AFAIK the model designation doesn't mean anything except possibly to the Japanese engineeers. The only one that may mean something though could be the original SR which was a pure racing engine. This was also their first Schneurle ported engine so it could stand for Schneurle Racing. Later engines were front induction but used the same crankcase in which case F might be Front induction but seeing they still have just SR cast under the exhaust port they were called FSR. It's the only way I can make sense out of it .