The introduction of mintmarks was primarily for quality control – to control actions of mint officials and help regulate consistent coin weights across the empire. Around the middle the 3rd century C. E., Roman mints began incorporating mint marks on their coins, also often the officina (workshop) that minted the coin. If a problem was found on a particular series of coins (underweight for example), the coins could be traced to the offending mint and the officials held liable. Mint marks are located on the reverse of the coin at the bottom in an area known as the ‘exergue’. The exergue is visually set off from the rest of the reverse design by a line. In some cases, part of the mint mark (such as the officina or workshop) may be found in the body of the reverse design or even on the obverse field. Being able to read the mints is very helpful in fully attributing a coin and is necessary if using RIC as your attribution source.
The Romans never established a consistent system for applying the mint marks and their mint marks contain one to four characters:
1st – a letter: P (Pecunia = money), M (Moneta) or SM (Sacra Moneta = Imperial money). Often these letters are omitted. In some mintmarks the abbreviated mint name is followed by PS (Pecunia Sacra). Mintmarks on gold coins often end with the letters OB (obryzium – refined or pure gold).
2nd – Mint city abbreviation (usually one to four letters, but up to seven).
3rd – a single letter indicating the Officina or workshop. Latin letter, a Greek letter or letters or a Roman numeral indicating the officina. In the Latin system, the officina was indicated by A = prima or 1st officina, B = secunda or 2nd, C = tertia or 3rd, etc
4th – Series marks. Many mintmarks also include symbols such as dots, a crescent, or a branch, for example. These symbols probably indicate when the coin was struck and who was responsible for the workshop at that time.
With the monetary reforms of Aurelian and Diocletian came changes in the mint markings [or at least the notation in the exergue – the area at the bottom reverse of the coin]. Roman numerals appeared, the meaning of which is still debated. Often a single letter or a letter between stars is all that appears in the exergue.
Various web pages devoted to Roman mint marks from:
The Collaborative Numismatics Project – Numiswiki