the easiest is to pull the prop drive washer- all the OS25s use a slip-on drive washer that will come right off. if it has a thick(approx .020) black steel washer behind it, and you can see a front ball bearing, it's BB. if the prop drive washer has a thin (just a few thou) stainless steel washer behind it, and you can see no front BB (you may be able to see the thin bronze bushing) it is plain bearing. another way is to remove the carb- the SF and FX BB engines use a 13mm carb neck, the plain bearing engines and the FSR BB use a 10mm- the FSR 10mm carb is dual needle, the plain bearing engines are airbleed carbs. also check the muffler bolt spacing- the FP and LA plain bearing engines use 32mm muffler bolt spacing, all the BB engines (FSR, FX, SF) use 35mm bolt spacings. the KP code on the engine lug is probably a factory lot or date code.