Hi John; The small letters you reffered to in your post are mint marks..... just like those mintmarks on U.C. coinage. It tells you which mint struck that particular token. The OPM stands for "Old Philadelphia Mint".... which is a private mint and has nothing to do with the U;S. Philadelphia Mint. Not sure what the "OC" mintmark stands for. Possibly Osborne? Many tokens have a "gd" mintmark which is a private minting company called "green duck". Earlier gaming tokens from the mid to late 60's will have "fm" mintmarks for the Franklin Mint. There are several other mintmarks designating other private minting companies which have struck tokens and medals. Archie