Mint marks on Roman coins go back to the first emperors. However, in the beginning they were simply used as extra space to put lettering. Starting late in the 3rd century mint marks become a tool for tracing and quality control. This was necessary following Diocletian's coin reform of the 280's as in his plan coins were to become uniform throughout the empire. Mints started by carving a letter or two into the exergue. Soon, not only the mint abbreviation but the internal offices, known as officinas, started appearing too.
Exergual marking can often be very difficult for the beginner because they seem to follow no predictable pattern. To determine which mint was responsible for striking a given coin just be on the lookout for a few key letters.
There were several major mints that operated throughout the 4th and into the 5th centuries. Rome, naturally, was active from the very beginning until its fall in 476.
Here is a list of major mints and their keys:
Antioch: ANT or ANA
Constantinople: CON or CONS
Heraclea: H or HERACL or HERAC
London: L or ML or LON
Lugdunum (Lyons): LG or LVG
Rome: R or RF
Siscia: SIS or SISC
Thessalonica: TES or TS
Ticinum: T Trier: TR
But it's rarely as easy as that. These abbreviations will often have other letters appended as either suffixes or prefixes that will be confusing. Add to that Greek letters and symbols such as dots, stars and other bizarre signs and it can often be daunting to make any sense of them. The task becomes simpler with familiarity... just look for the key letters embedded somewhere in there to point you in the right direction.
Note: When reviewing the database you will notice that many entries have an "officina" within parentheses. This is to signify the correct placement of an officina letter in relation to the rest of the exergual markings and should be considered as a generic placeholder. Officina letters do not normally affect the attribution of a coin so that, for example, a coin with a mintmark "SMTSB" and another with "SMTSA" will both have the same RIC and/or LRBC attribution assuming all else is the same.