Here are the two different types of 15 rouble coins that I own. Both are so-called “large head” varieties. All of the “small head” varieties are really pattern coins and extremely rare; only the Kazakov catalog lists them as such, however.

The “СС” type has the legend on the obverse extending past the neck of the portrait by just two letters, whereas the “ОСС” type (sometimes called “РОСС” type) extends by 3-1/2 letters. These correspond to the Kazakov catalog numbers 63 and 66 (the latter is the somewhat scarcer “wide rim” variety).