I just posted this in response to a question on the ID board, but thought I'd put it here also since it may be generally useful, and is less likely to be buried/deleted.
The pattern for mintmarks during this time (Constantine era) is:
- In the latin speaking west (Italy west of Ticinum, and west of Italy), latin officina letters are used (P/S/T/Q, etc)
- In the greek speaking east (east of Italy), greek officina letters are used (A/B/gamma/delta, etc)
Officina position, when in the exergue, also goes by geographical location. The pattern outside of the central region (Italy east of Ticinum & Pannonia) of the empire is:
- In the west, the officina letter is at the start (= western end!) of the mintmark (e.g. PTR, STR)
- In the east, the officina letter is at the end (= eastern end!) of the mintmark (e.g. SMANTA, SMANTB)
In the central region, the placement is reversed:
- In Italy east of Ticinum (i.e. Aquileia, Rome, Ostia), the officina letter is at the END of the mintmark (e.g. RP, RS)
- in Pannonia (Siscia, Sirmium), the officina letter is at the START of the mintmark (e.g. ASIS, BSIS)
There are only a couple of exceptions to this:
- Arles, always a bit of a maverick mint, usually uses the expected latin officina letter at the start of the mintmark (e.g. PARL, PCONST), but occasionally latin at end (ARLP), and even greek at end (ARLA).
- Aquiileia under Maxentius used the irregular mixed latin/greek officinas P/S/gamma, but this was regularized under Constantine to the expected P/S/T.
When you see such a clear and logical pattern, it makes it extremely unlikely that any undiscovered mintmark would not at least stick to the pattern, and in practical terms the chances of finding an undiscovered mintmark (as opposed to just an unlisted officina letter) is very small, which is why it is best to treat mintmark matching as a "multiple choice" question from the known mintmarks for the mint+type you are looking up (e.g. Siscia VRBS ROMA, or Thessalonica VRBS ROMA). If you think you have found a mintmark (vs officina letter) that is not in the reference books then unfortunately it most likely means that you have misread the mintmark!