Cletus,

It sounds like you have one of the very old O.S. Max .25's. We'd not know where you might be able to find out the exact model.

You might want to check the piston. If it has an upright "fence" on the top, you have one from the 70's or earlier. If it's a .25 FSR, then there'll be an "F" or an "SR" on it somewhere.

The letters on the bottom of the mounting flange are the code O.S. uses to identify the production batch. If your's is old enough, even they wouldn't know which batch your engine came from.

If you can post a pic, we can give you a better idea which engine you have.