Tried this in the Amp forum, but the only response I got was to try here, so here goes:

I'm looking into modding my Fender Pro Jr., and the mods I've found vary slightly depending on which revision of the circuit board it is, and I was wondering how to tell which revision you have? The Serial Number starts with "B-#######" so it would make sense if I had revision B, but it seems that revision B would place it in the early 90's, and from the condition of the amp, I did not believe it was that old. I'm not the original owner, so it's entirely possible that it is revision B, but I would just like to be 100% sure before I start making a mess of things :-P

If you can tell the difference between the mods, I'm sure you can tell the difference between the pcbs by looking at them.