Coin Relief – Issue Seventeen

Welcome to the next edition of Coin Relief. In this issue Dr. Andrew Brown looks at coinage from the period of the Tetrarchy. The Tetrarchy was a system of government instituted by the emperor Diocletian in AD 293 to try and bring some stability to the Roman Empire. Under this system, the government of the empire was divided between two senior emperors, the augusti, and their juniors and designated successors, the caesares.

The Tetrarchy, AD 293-307

In AD 293, the diarchy established almost a decade earlier by Diocletian (AD 284-305) that saw him rule in the east with his
co-augustus Maximian (AD 286-305) in the west expanded to a tetrarchy of two senior and two junior rulers. Galerius (AD 293-311) assumed the title of caesar in the east under Diocletian, while in the west the nobilissimus Caesar was Constantius I (AD 293-306). The first task of the latter was, of course, the restoration of Britannia to the Roman empire following the usurpation of Carausius and then Allectus between AD 286-296. Diocletian, and by extension Galerius, was styled as the son of Jupiter – Jovius – Maximian as the junior partner, and his junior Constantius, on the other hand, assumed the title Herculius claiming descent through Hercules rather than Jupiter. Not only did this perhaps give them an element of divine legitimacy, but it also created an Imperial college and a more stable administrative structure from which to rule the empire, with each partner adopting characteristics, positions, and roles based on their Jovian or Herculian descent.

In this edition we will look at the coinage following the establishment of the Tetrarchy and Diocletian’s reforms of the coinage in c.AD 294 until the death of Constantius I and elevation of his son, Constantine I (AD 306-337) in AD 307. The history of the Tetrarchic period isn’t always straightforward, but the coinage at least has some basic elements that are commonly seen through the PAS. The standard reference for this period when recording coins through the PAS remains RIC VI.

Diocletian’s reforms, c.AD 294-301

Aurelian (AD 270-275) had attempted to stabilise Roman currency and inflation within the fractured empire in the AD 270s, but had not met with complete success. It was not until Diocletian, who introduced several reforms beginning in c.AD 294, that greater stability was exerted and with some control over coin production empire-wide. 

Diocletian’s currency reforms in c.AD 294 re-established a trimetallic system of coinage and introduced new denominations into circulation. We have already seen in previous blog posts that the silver argenteus emerges at this time but perhaps more significantly for the 4th century is the appearance of a large silvered bronze coin, the nummus. In the initial period of reform the coinage structure, based on relative values to the denarius communis (hereafter ‘d.c.’; now essentially a unit of account rather than an extensive coinage in circulation), saw the following coins in use:

Image: Andrew Brown.

A second piece of legislation was issued from Diocletian’s capital in Nicomedia and was enacted on the 1st of September AD 301. This currency edict is partially preserved in the Aphrodisias currency inscription (Aphrodisias, Turkey) and establishes the denominations and their relative values in d.c. in use at that time. The precise purpose of the edict is a matter for discussion. It appears to have worked in tandem with the Edict on Maximum Prices (See below) to impose economic control, perhaps to curb inflation, prevent the hoarding of the new higher value denominations, and to benefit the Roman state notably in its recouping of taxes and bullion or for military pay and expenditure. For the coinage, the argenteus and nummus appear to have initially been undervalued relative to their metal content and the Currency Edict effectively doubled their face value with the coins themselves remaining the same. Thus, the argenteus after AD 301 was tariffed at 100 d.c. while the nummus increased to 25 d.c. and the post-reform radiate to 4 d.c.

About 140 fragments from inscriptions that preserve the Edict of Maximum Prices have been identified at various locations in the Roman world, mostly in the eastern half of the empire. This was introduced a few weeks or months after the Currency Edict, perhaps in November or December AD 301, but the two seem to have worked together as part of the same economic reforms implemented by the Tetrarchy. Where the currency edict
established the values of the coinage, the Prices Edict attempted to curb inflation by fixing the maximum prices that could be charged for commodities, goods, and services empire-wide. The edict itself notes the affect unregulated prices, which varied regionally, had on the compulsory purchases required for the military. It goes on to list over 1,200 goods based on their commodity value, services (e.g. salaries, daily wages, etc.), and the
value of bullion. Thus, there is everything from the price of wheat per modius at 100 d.c., to a sextarius (pint) of wheat beer (4 d.c.), a fattened pheasant (250 d.c.), 10 dormice (40 d.c.), a bundle of small parsnips (6 d.c.), 2 d.c. per customer for a barber, or 25 d.c. a day for a sewer cleaner or an armorer to sharpen a sword. At the other end of the spectrum, 80 d.c. for Babylonian purple slippers, 1,500 d.c. for a top-quality leather travelling bag, 6,000 d.c. for a hooded cloak from Britannia (a birrus Britannicus), or 150,000 d.c. for a pound of unprocessed silk dyed purple! The bullion value of gold is set at 72,000 d.c. per pound.

These were the maximum prices that should be charged around the empire for each good or service listed. However, the experiment with state control in this manner was ultimately unsuccessful. It did not account for the economic differences from one end of the empire to the other and indeed met with resistance and even black-market trade.

Tetrarchic coinage, AD 294-307

Against this background of monetary and economic reform, control of coin production was also tightened. A network of sixteen mints operated during the Tetrarchic period, many continuing from preceding decades, but others (Aquileia, Thessalonica, and Nicomedia) established with Diocletian’s reforms and others still (Ostia, Serdica, and Carthage) appearing intermittently. By AD 301 this had resulted in standardised production around the empire and the removal of Roman provincial issues. Mints now marked coins with mint and officina marks to ensure their quality and, perhaps more notably, coin types and legends (in Latin) were standardised around the empire so that, for example, a nummus from a western mint should be essentially the same as one from an eastern mint with the exception of its mintmark. The mints operating during the Tetrarchic period were:


The PAS records c.5,500 coins dated to the Tetrarchic period of Reece Period 15 (AD 296-317), with an additional 6,799 IARCW Welsh records. In this blog we will look at the c.5,500 coins alone, since the Welsh data contains a mixture of hoard, site, and single coin finds and lacks images to verify the identifications. This will be divided into two broad sections, roughly corresponding to the First, Second, and Third Tetrarchies between AD 294-307 and the elevation of Constantine I to Augustus.

Aureus of Maximinus II, c.AD 305-308. Record ID is BERK-E53380 (image: CC-BY). 

Coinage struck in gold and silver is rare in Britain for the Tetrarchic period. A single gold coin, an aureus of Maximinus II as caesar, has been recorded through the PAS, while the two argentei of Diocletian on the database have been looked at in previous blogs. A handful of other examples post-dating AD 307 are noted on the PAS and will be looked at in a future edition, but in total there are no more than 19 silver (and base silver/billon) and gold denominations for the entire period up to AD 317. 

Only 27 radiates struck following Diocletian’s reforms in AD 294 are recorded on the PAS. These are not common as British finds given their production in eastern mints. In any case, Britain remained separate from Rome until AD 296 while first Carausius and then Allectus presided over a breakaway empire and large quantities of radiates (official and otherwise) remained in use here, perhaps integrated later on with the new monetary system. The nummus does not appear until after Constantius reclaims the province for Rome.

Post-reform radiates of Diocletian (SWYOR-723F23), Constantius I (BERK-061FF7), and Maximian (SUR-4E5E51), all license: CC-BY.

The Post-reform radiates recorded through the PAS are products of just five Roman mints (see table below). They can be difficult to distinguish from later pre-reform radiates, but lack the XXI usually seen in the exergue of earlier coins of the Tetrarchs, while some carry vota reverse types that are distinctive.While the PAS coins appear to be genuine finds, it is possible that some may be more recent rather than genuine ancient losses – as with any unusual coin, an accurate findspot and decent image is vital in this regard.


By far the most common coin type, and the standard in Britain for this period, is the large (originally silvered) nummus introduced by Diocletion in AD 294. The earliest examples are large and can often be separated from later coins even when quite worn or corroded due to their laureate busts and large module – be careful not to confuse with very worn early bronze asses though (the mintmarks and lack of S C on the reverse are a giveaway here)! There are at least 1,400 nummi dating to the period between AD 294-307 recorded through the PAS. I think this number is liable to change up or down – there are a significant number of coins recorded within the broad Reece Period 15 range that could be potentially dated more closely. This is something to remember when recording the nummi of this period through the database, it is important to use the date ranges of the coin issue (where known) rather than the regnal dates of the emperor for the date ranges.

The nummus introduced by Diocletian has one standard type that is employed empirewide and which represents the bulk of the material seen in Britain. This is the distinctive GENIO POPVLI ROMANI (‘to the genius of the people of Rome’) reverse depicting Genius standing left holding patera and cornucopia (Figs.). If you are given for recording a large bronze coin with this reverse type it is probably going to be a Tetrarchic coin.

First Tetrarchy, AD 294-305

Nummi of the First Tetrarchy. Left to right: Diocletian, c.AD 303-305, Trier mint (WILT-2F2216); Maximian, c.AD 300-303, London mint (SF-022D13); Galerius, c.AD 301-303, Lyon mint (LON-EBC4AF). Images: all license CC-BY.

The initial phase of Tetrarchic rule, the First Tetrarchy, until AD 305 sees a relatively uniform corpus of material recorded through the PAS. With Diocletian and Maximian as the senior augusti and Galerius and Constantius as their juniors, there are at least 1,090 nummi recorded. Over 90% of these where the reverse type is legible are the standard GENIO POPVLI ROMANI types, with coins from twelve of the Roman mints so far identified. As might be expected, the western mints are the most regularly seen, with Trier taking the lead, followed by London and then Lyon. Maximian is the most commonly represented ruler, followed by Diocletian and then the juniors Constantius Galerius. 

Although the GENIO POPVLI ROMANI type dominates, it was by no means the sole nummus type issued. Other examples are quite rare on the PAS, although distinctive Moneta types and coins issued from Carthage do appear. 

Left: Moneta type nummus of Diocletian (DOR-D9378D). Right: Carthage nummus of Constantius (ASHM-6C2861). All images: license CC-BY.

Second and Third Tetrarchies, c.AD 305-307

Diocletian had fallen ill while at Nicomedia in AD 304 and the following year, something extraordinary happened. Galerius travelled to Nicomedia and many thought Diocletian had died with Galerius preparing to assume power. However, this was not the case. Instead, on 1st May AD 305, Diocletian called an assembly on the hill outside Nicomedia where he had been proclaimed emperor in AD 284 and publicly became the first Roman emperor to abdicate power and retire. Maximian followed suit and so the two junior partners, Galerius and Constantius, now assumed power. Although it was expected that Constantine I (son of Constantius) and Maxentius (son of Maximian) would be the next in line as junior partners in the Tetrarchic college this turned out not to be the case. Galerius it seems instead assured the appointment of Severus as caesar in the west under Constantius and Maximinus Daia under his control in the east.
Things were to change again just a year later. Constantius controlled the western provinces and was responsible for Britain and Gaul. While campaigning in northern Britain against the Picts, he fell ill and recalled his son to York where, before his death on the 25th July AD 306, conferred power on Constantine (who accepted position as caesar but not augustus!) and in so doing side-stepped the natural progression that Galerius had
manufactured for the Tetrarchy. Galerius accepted Constantine’s place in the Tetrarchy as caesar in the west, with Severus elevated to augustus. It was later in AD 306 that Maxentius, with the support of Maximian who came out of retirement, usurped power in Rome. Severus was ordered by Galerius to march on Maxentius, which he did the following year, but in defeat and captured at Ravenna he was removed from power. By July AD 307 Constantine assumed the title of Augustus. But the structure of the Tetrarchy established by Diocletian had clearly broken down.

Nummi of the Second and Third Tetrarchies. Left to right: Severus II, c.Ad 305-307 (HAMP-B6307E); Constantine I, c.AD 307 (LIN-C74D93); Diocletian, c.AD 305, peaceful retirement type (QVIES AVGVSTORVM) (HAMP-A4451A).

The coinage of this period of political change is much less extensive than the First Tetrarchy, as might be expected given the two-year period between Diocletian and Maximian’s retirements and Constantine assuming power as augustus. The PAS has approximately 300 examples identified to date, not including those coins of Maxentius that post-date Constantine’s elevation as augustus and which we will look at in a future edition. The standard GENIO POPVLI ROMANI type remains in circulation, however with Constantine I and Maxentius, the nummus shrinks to 1/48 of a pound by AD 307 and drops in weight – the beginning of the decline of the large module coin to the smaller copper-alloy coins seen from the AD 330s onwards. This results in the legend shrinking to simply read GENIO POP ROM. For the retired augusti, Diocletian and Maximian, obverse titles adjust to reflect their new status as senior augusti (P F S) and reverse types point to their restful or peaceful (QVIES) retirement.

The Tetrarchy, AD 307-317

The second half of the Tetrarchic period covered by Reece Period 15, between AD 307-317, is a complex phase of Roman history and politics – there isn’t enough space here to do it justice! However, it is worth looking at and briefly summarising for the coinage that we see through the PAS. We have already seen how, after the retirement of the senior augusti Diocletian (AD 384-305) and Maximian (AD 286-310), Constantius I (AD 293-306) ruled in the west with Severus (AD 305-307) as caesar, with Galerius (AD 293-
311) and Maximinus Daia (AD 305-313) in the east. Constantius’ death at York in July AD 306 resulted in the legions there declaring for his son, Constantine I. Galerius instead promoted Severus to augustus but acknowledged Constantine’s position as caesar. Constantine initially seems to have accepted his role as junior partner and controlled the western provinces of Britain, Gaul, and Spain with coins struck from this period until he assumed the title of augustus in July AD 307 naming him nobilissimus caesar.

Nummus of Divus Constantius I, c.AD 307-310 with MEMORIA FELIX reverse (BUC-08E7B1, copyright: Buckinghamshire County Museum, license: CC-BY).

There are approximately 45 divus constantius nummi recorded through the PAS that commemorate Constantius’ death. All are of the MEMORIA FELIX type except the single example struck under Maxentius at Ticinum noted above.

AD 307-313

After Severus’ defeat at the hands of Maxentius, Constantine styled himself as augustus. In Italy, Maximian came out of retirement in late AD 306, first in support of his son Maxentius’ usurpation against Rome (AD 306-312), then in an attempt to usurp him in AD 307, before fleeing to Constantine in the west where he again attempted to usurp power before committing suicide on Constantine’s orders in AD 310. There are over 100 post-retirement nummi of Maximian recorded on the PAS. About a dozen of these are of the Providentia type, but the majority are the standard nummus with shortened GENIO POP ROM reverse legend and type.

A post-retirement nummus of Maximian, 305-307 (PUBLIC-CB6BC2, copyright: Portable Antiquities Scheme, license: CC-BY).

At a council in Carnuntum (Austria) on 11th November 308, Diocletian came out of retirement and met with Maximian and Galerius to try and stabilise the Tetrarchy, where it was decided that Maximian should retire (again!) and that Licinius I (AD 308-324) should replace Severus as augustus in the west. This was clearly not ideal for either Constantine I or Maximinus Daia, who were both overlooked. Upon Galerius’ death in AD 3111, Licinius
extended his control eastwards into the territories of Maximinus Daia, with Maxentius sandwiched between him and Constantine I in the west. Only a single silver coin, a rare quinarius of Constantine I, is recorded on the PAS in the period immediately after Constantine becomes augustus in AD 307. Almost all of the coins recorded up until AD 313 are nummi – there are more than 1,300 examples identified to date, although I suspect this number is potentially slightly higher since the two standard types are often difficult to date more closely than the later Tetrarchic period when poorly preserved.

quinarius of Constantine I, c.AD 307-308 (FASAM-F1A323, copyright: All Rights Reserved, License: CC-BY).

The nummus underwent a series of reductions between AD 307-313 instigated first by Constantine in the west before spreading east and including the mints operating under Maxentius’ control. By AD 310 it weighed between 4-5g and continued to decrease in size, particularly after AD 317 when it halved in value. An interesting group of nummi struck at Lyon in AD 308-309 and the eastern mints of Nicomedia (in AD 308-311) and Cyzicus (in AD 311-312), carry marks that reinforce the nummus’ face value after the initial reductions. Thus, coins from Lyon are marked CI HS to indicate their value at 100 sestertii, the equivalent of 25 d.c.; while those from the eastern mints have reverse legends that end CMH for the same value but with ligated Greek numerals MH. These are not common as PAS finds, although at least 6 examples from Lyon and four from the eastern mints are recorded. Alongside the weight reduction in the nummus was the introduction of fractional denominations – half, third, and quarter (or less) nummi. More than 130 examples have been identified on the PAS so far with the majority dating to the period c.AD 310-311 and from the mint of Trier.

A range of nummi are struck after AD 307, typically with reverse types that have associations to Mars, Jupiter, Sol, and Genius, amongst others. By far the most prevalent, however, and important to recognise if you are recording coins of this period in Britain, are the GENIO POP ROM and SOLI INVICTO COMITI types that are produced in large volume particularly from the western mints. Coins issued from the eastern mints are far less commonly seen, although do turn up notably with Jupiter reverse types.
The GENIO POP ROM reverse type is prolific at most mints, and a continuation of the longer GENIO POPVLI ROMANI types on the larger nummi of the early Tetrarchic period. 

Nummus of Constantine I, c.AD 308-309, with GENIO POP ROM reverse (LEIC-67DA0B, copyright: Leicestershire County Council, License: CC-BY).

There are approximately 500 examples identified to date on the PAS (many without images), with examples from several mints across the empire although largely confined to issues from the western mints of London and Trier. For the period up to AD 313 almost half of the examples are identified as products of the London mint and a third from Trier. This switches in the subsequent period between AD 313-317, with almost 60% from the Trier mint and 35% from London. It should be noted that this number is liable to change in either direction as there are at least 60 examples that do not have mint attributions and many others may need adjusting.

By far the most common type in this period, however, is the SOLI INVICTO COMITI nummus for Constantine I, which experiences a large volume of production particularly after AD 310. On his way to Trier to celebrate his quinquennial year, Constantine is reported by the anonymous orator of the panegyric (VII, 21.3-6) in AD 310 to have stopped at a temple where he had his first religious vision of “Apollo with Victory accompanying him, offering you crowns of laurel, each one of which brings you an omen of thirty years”. After this, Sol Invictus, the invincible sun god, becomes Constantine’s protector and is most frequently styled as SOLI INVICTO COMITI as the invincible companion of the emperor. Coins issued with Sol types, particularly from the western mints, are prolific during this period, with over 2,500 PAS examples between AD 307-317. Of these currently with mint attributions, the London and Trier mints each make up about 40% of the total before AD 313, with London rising slightly to c.45% and Trier just a quarter of the total up to AD 317. As with the Genio types, these figures are likely to change! What is less likely to change is that fact that coins depicting Sol in this period are almost always for Constantine (about 90% of the total), with very few examples for the other emperors. It is worth noting that although Sol standing holding a globe is the most common type, he also appears on fractional nummi with abbreviated SOLI INVICTO reverse legend and on coins that have a radiate bust of sol on the reverse. Of the latter, there are about 30 examples on the database so far.

Nummus of Constantine I, c.AD 313-314, SOLI INVICTO COMITI reverse (HAMP-3AA312, copyright: Winchester Museums Service, License: CC-BY).

The standard SOLI INVICTO COMITI type continues until AD 317, so do check the mintmarks if visible to identify whether your coin falls early in the period or towards the end. Several other less common types do appear in this period too, notably depicting Mars. The Jupiter types from the eastern mints are not common as PAS finds, with fewer than 30 examples recorded to date. The Mars bust type should be readily distinguished from the Sol type by the presence of a helmeted rather than radiate bust on the reverse. These portrait reverse types are struck at Trier only.
Several rarer issues are also recorded through the PAS. These include two nummi of Galeria Valeria, the daughter of Diocletian and second wife of Galerius. During his usurpation, Maxentius issued coinage from the Italian mints and there are 34 coins on the PAS struck in his name from Rome, Ticinum, Ostia, and Aquileia. These are mostly of one reverse type with CONSERV VRB SVAE legend and hexastyle temple, but also includes a single example of a posthumous issue of Divus Romulus, Maxentius’ son struck in AD 309.

Nummus of Galeria Valeria, c.AD 310-311 (WMID-32E0F4, copyright: Birmingham Museums Trust, License: CC-BY).

AD 312-313

AD 312 and 313 brought about three key events. In October AD 312, Constantine defeated Maxentius at the Battle of Milvian Bridge, thus ending his usurpation and giving Constantine control in the west. Maxentius had declared war on Constantine after his father, Maximian’s death and, in fear of Constantine allying with Licinius, Daia threw his support behind Maxentius. North of Rome near the Milvian Bridge, Constantine’s forces faced those of Maxentius, which were significantly larger. It was here that Constantine received his second divine vision – according to Eusebius (Vita Constantini I.28; see also Lactantius De Mortibus Persecutorum XLIV.3-9) this took the form of a vision at about noon of a cross of light above the sun accompanied by the inscription (in Greek ἐν τούτῳ νίκα) ‘in hoc signo vinces’ (‘in this sign, you shall conquer’) – prompting him to direct his troops to mark their shields with Christograms. The battle, on the 28th October 312 was decisive and Constantine reached Rome in triumph the following day while Maxentius’ body was fished out of the Tiber! Several coin issues struck after this date demonstrate Constantine’s authority in Rome and his restoration of peace following the victory.

Left: Nummus of Constantine, c.AD 312-313 (OXON-A8CCA9, copyright: Oxfordshire County Council, license: CC-BY). Right: Fractional nummus of Constantine I, c.AD 313-15 (WILT-C47400, copyright: Portable Antiquities Scheme, license: CC-BY).

Early the following year, Constantine met with Licinius in Milan. Here Licinius married Constantine’s half-sister, Constantia, to secure an understanding between the two men, at which time they also issued the Edict of Milan that promoted religious tolerance and prevented the persecution of Christianity within the empire. Maximinus Daia had sided openly with Maxentius and he was soon defeated in Battle by Licinius at Heraclea before fleeing to Tarsus where he died in August AD 313. This left the empire effectively divided in two by the end of AD 313, with Constantine (who championed Christianity) in the west and
Licinius (a pagan who tolerated Christianity) in the east. The period between AD 310-315 presents some interesting material with regard to the coinage recorded through the PAS. A series of coin issues for Constantine from the mint of London depict Adventus reverse types – the ‘arrival’ of the emperor. One issue in AD 307 likely relates to a visit by Constantine to Britain in that year. A larger group of coins in the period c.AD 310-312 plausibly relate to another imperial visit by Constantine, perhaps in advance of Milvian bridge, for which he could well have recruited forces in Britain (although RIC VI, p. 121 sees this issue as representing Constantine’s threatened arrival in Rome).

Finally, a third group dating to AD 313-315 might point to a third visit to the province at some stage during that period. This at first glance doesn’t seem hugely out of the ordinary. What is fascinating, however, are the other coin types that appear contemporary with these issues. Gold coinage begins to reappear through the introduction of a lighter solidus struck at 1/72 lb in c.AD 309/310. A single solidus of Constantine – of a type previously unrecorded in Britain – was recorded through the PAS in 2019 and dates to AD 313-315 (Fig. 36). This is joined by two multiple aurei (1 ¼ aurei, sometimes termed festaurei), both of Licinius, dating to the same period. A further contemporary solidus from Britain is noted by R. Bland and X. Loriot (no. 229) along with another multiple of Licinius (no. 169). 

Gold and silver! Left to right: Solidus of Constantine I (SOM-A557C8, copyright: South West Heritage Trust, License: CC-BY); Festaureus of Licinius I (WILT-D86FB6, copyright: Salisbury and South Wiltshire Museum, License: CC-BY); Argenteus of Licinius I (WILT-369E64, copyright: Salisbury and South Wiltshire Museum, License: CC-BY).

In addition, between AD 310-313 we start to see the reappearance of silver in the PAS data with silver argentei for Constantine, Licinius, and Daia along with their base or billon copies. This coincides, too, with the spike in production (between AD 310-313) and loss of fractional nummi in Britain at around the time of celebrations for Constantine’s 5th year in power. As we have already seen, these fractional nummi do not appear in hoarded assemblages in huge volume as part of the regular currency in circulation and the suggestion instead is that they may have acted as donatives dispersed during the commemoration of specific Imperial events. The same may also be the case for the larger gold denominations, which could be seen more as a medallic coinage rather than currency in circulation. Sam Moorhead has already made the suggestion that this series of special gold coins struck at Trier between AD 313-315 for both Constantine I and Licinius I, with known museum examples and now findspots through the PAS finds in Britain, may have been shipped to the province in a batch as part of imperial largesse during a potential visit in AD 313-314. I think it possible that the combination of Adventus types, fractional nummi, silver and base argentei, and gold denominations, all of which cluster around the two broad dates of AD 310-313 (second group of Adventus coins, fractional nummi and argentei) and 313-315 (third group of Adventus types and gold) could well correspond with visits to the province by Constantine, or at least planned ones. Is it possible that the AD 310-313 group relate to Constantine being in Britain prior to Milvian Bridge (perhaps to garner support and manpower)? And could the second batch between AD 313-315 provide evidence for a third visit after Constantine secured his position as emperor? We perhaps need more material to substantiate this fully, but it is an interesting possibility!

AD 313-317
An uneasy period of truce followed the meeting at Milan, before civil war ensued and the armies of Constantine and Licinius met at Cibalae (Croatia) forcing Licinius to flee to Sirmium. A further Constantinian victory followed at the Battle of Mardia (Bulgaria) in AD 316/317 with a peace concluded on 1st March AD 317 at Serdica. This was effectively the end of Tetrarchic system established by Diocletian, since the two augusti promoted their sons (Crispus and Constantine II for Constantine and Licinius II for Licinius) to caesars and reestablished a dynastic system of rule.

Nummus of Constantine II caesar, c.AD 317 (NMS-FDF034, copyright: Norfolk County Council, License: CC-BY). 

The coinage of this period differs little from the preceding phase on the PAS, although there is far less variety in the types (Fig. 40) seen and now the Sol types for Constantine – who is after all now in sole control in the western empire – dominate. The London mint again accounts for the largest share of those coins currently given a mint attribution on the PAS. At the very end of the period, in AD 317, a few slightly less common types for Constantine II as caesar appear and are worth looking out for if you are recording coins on the database.

References and further reading:

R. Abdy ‘Tetrarchy and the House of Constantine ‘in The Oxford
Handbook of Greek and Roman Coinage 2012

G. Bransbourg ‘Inflation and monetary reforms in the fourth
century: Diocletian’s twin Edicts of AD 301’ in K. Butcher (ed.) Debasement: Manipulation of Coin Standards in Pre-Modern Monetary Systems (Oxbow, 2020): 165-194

The best source for the London mint coinage of this period is H. Cloke and L. Toone, The London Mint of Constantius and Constantine (London: Spink, 2015)

A key reference for the mint of Lyon is P. Bastien Le Monnayage de l’atelier de Lyon (6 vols.)

For Maxentius, see V. Drost Le monnayage de Maxence (2013)

R. Bland and X. Loriot Roman and Early Byzantine Gold Coins found in Britain and Ireland (Royal Numismatic Society Special Publication no. 49, 2010)

S. Moorhead and D. Stuttard, The Romans Who Shaped Britain (2012): pp. 194, 196