I wish I knew the 'system' Rome mint used for some of their mintmarks but I don't and I have seen nothing suggesting anyone alive in the last 1000 years has. R P is easy since w can guess R would be for Rome and the choices P, S, T etc. point to the first letters or ordinal numbers primus, secundus and tertius. the question is what the B or the E or the star added to the matter in their series and why we dont see, for example, RAP, RCP or others. This is not limited to just the tetrarchy since Rome got creative later in the FEL TEMP period as well. As far as the A=R etc. goes, I suggest we all take the attitude it is our job to learn the language on the coins. I don't mean Latin but learning the type faces favored in different places at different times. The long dead are unlikely to change the way they did things to match your idea of proper lettering. A few Rome mint coins: Maxentius RET Maxentius RBT Jumping ahead 35 years or so: Constantius II R.F.E R.F.Q RMP* This last M might be the same as coins with M higher in the field which have been guessed to mean that the tiny bronzes are 1/1000 of a gold solidus. I'm not saying that this theory is right or wrong but just that there is a lot we don't know about Roman coins when it comes to questions starting with 'Why?'. Why did Rome stick a letter in the middle? What does it mean? Let me know when you figure it out. I challenge someone now in their teens to do better in 50 years than my generation has done. It is not all bad. Scholars have figured out the three below from Probus' Roman series. We have discussed them here before so I'll just give the link. I really should seek out the rest of the set. Anyone have them? http://www.forumancientcoins.com/dougsmith/equiti.html