So it appears that at Trier the mint master changed the field letters for the 2nd issue, added gamma (Greek 3) for the third issue, but chose star for the fourth issue. No rules - do what you want....
Well, certainly a lot of variety as far as issue marks goes, but the addition of the
officina designator does appear to have been coordinated across mints, even if the precise form of it was not.
Diocletian's initiial specification for
nummus markings seems to have been just to include the city name (e.g. TR for Trier, R for
Rome, etc), which is what we see on all initial issues. There then appears to have been a follow-on order to add an
officina designator, which appears at most mints on their second issue. The exceptions are
Antioch and
Alexandria who used
officina designators from the get-go, possibly due to not having started production until this follow-on instruction
had been received.
The
form of the
officina designator does vary a
bit. At most mints it was a greek letter (Alpha, Beta, etc). The obvious exceptions are
Ticinum and
Aquileia which went with latin numbers instead (Prima, Secunda, etc).
Rome might have been expected to group with the other
Italian mints, but instead stuck with greek.
The only other exception for form of
officina designator are these Trier A/B/C ones, which can only be latin letters (not greek). It's interesting to wonder if this was done deliberately, or conceivably an error based on unclear instructions.