RE: OS 10 FP,FSR, LA etc
Basically, they're all fairly similar. They were plain-bearing, non-ringed engines. All have Schneurle porting, which is where the .10 FSR got the "SR" part of its designation. It was named the "FSR" to distinguish it from the Pet .099 that was a crossflow ported engine.
The .10 FP got its designation because it was styled to conform to the FP-series of engines. And the .10 LA was changed in the same way. They are all very close in power and operation. The .10 FSR and .10 FP can be told apart by looking at their castings. It will have the designation molded into it.