Bax,your almost completely correct. Only 1 small mishap,that everyone makes. I have the OS complete tare down and manufacturer's labeling for pretty much,every OS engine up to 1991. The Letter's following all the FSR series engines,actually is a acronym for short. The F stands for FOUR,the number of ports in the case. The S is for Schneurle the type of porting. Last but not least is the R,it stands for RINGED only,not rear.

Every FSR engine is a Ringed engine,unless it was modified,which i have seen people do to make the ABC version.

Good lying all Art