I have 3 different OS.25. Two I know what they are for sure, a .25fp and a .25f. the third is SUPPOSE to be a .25fsr. On the case all it has is the vertical os max and the # 25 in a circle. I bought this at a swap meet in a .25fsr box, hope I wasn't duped.

Thanks Steve

Does it have ball bearings?

The 25 F and the 25 FSR are pretty much the same engine.  The 25 F is ABC or ABN, and the head fins are just a hair taller.  The ABC construction makes for easier hot restarts and a little more RPM on the top end.  It's also a little more consistent.  The difference between the two is similar to the difference between an iron piston FP and an ABC (ABN) FP. 

I have several of both the FSR and the F.  Both were run on a tuned pipe on a Primary Force ARF, and a Nemesis combat plane, with decent results (high RPM).  I always end up using the F over the FSR for the reasons I mentioned before.  With a wide open intake, bladder, no muffler and an 8" prop the 25F carried the Nemesis around the circle at around 93mph.  With the pipe, and a small venturi, the same engine was extremely consistent and carried the P-force like a good .40.