In addition to the mint mark, a workshop (also called the officina) mark may be present. Many cities would have several workshops where the coins were minted, and these workshop letters can be used to determine which one made the coins (something that would have been useful for the Roman administration if the quality of metal wasn’t pure enough etc). Archaeologists actually find ancient mints once in a while, with piles of coins bearing the same workshop letter, hence, you might be able to track down exactly where your coin was made!
A mint mark on a coin may be preceded by the letters P (Pecunia = money), M (Moneta = Juno Moneta, goddess of coinage), or SM (Sacra Moneta = Sacred/Imperial Money). SM is the most common use, with P and M being much less common. Finally, following the city mint mark (but sometimes preceding, if the M or SM are absent) one can often find a single letter, which designates the workshop. The standard coding for the workshops of the Eastern mints are Greek capital letters which stand for 1st, 2nd, 3rd, 4th, 5th etc. workshop, while for the Western mints Latin letters were used – P, S, T, Q and V standing for Primus, Secundus, Tertius, Quartus and Quintus (again, 1st, 2nd, 3rd, 4th and 5th workshop).
Higher number workshops existed, but are not very common. These letter can quite often be found in the field, that is, somewhere around the main design of the coin, but not at the bottom. See bottom of Table 7 for all these.
For the Antoniniani, the silver content by the end of the 3rd century was 5%, which was sometimes denoted on the reverse of the coin by the Roman numerals XXI, or XX, meaning 20:1, or 20 parts of bronze to one part silver. See for example the coin of Aurelian in Figure 15. Some mints only used the XXI and the workshop designation, which makes it difficult to tell which city the coin comes from – another case where some research on the web will be required, or a look into RIC. The Greek letters KA (=21) were also used for this purpose in some mints, Serdicia being by far the most common. Hence, a mint mark of KAA is for Serdicia, 1st workshop.
In addition of the regular mint marks, one can also find other letters in the field (the area around the reverse design) of 4th century coins. These are called control marks, and were used by a mint possibly to trace the coin to the exact place of production within a mint, or persons responsible for their minting. Their exact meaning and use is not fully understood. Control marks are fairly frequent on early 4th century coins, and especially those of Constantine the Great. See for example Figure 17, on which the letters S F are found in the field.
A rare case of the use of a workshop number can be found on some coins of Constantius II. These use the letters OF I or OF II to designate the officina (1st or 2nd workshop in this case). This practice was not widespread however.