Blank engine cases was provided with no stampings. I submit that your engine was pulled from a '64 frame and used elsewhere. As in your case it was installed in the '68 RS frame. I do not believe an engine going down the normal production line (typical engine stamping) would have been built any differently than any other engine of similar production intent.

1967 BSA Wasp
1967 BSA Hornet (West Coast Model)
1967 BSA Hornet (East Coast Model)
1968 BSA Firebird Scrambler
1968 BSA Spitfire Mark IV
1965 BSA Cyclone Competition Build
1965 BSA Spitfire Hornet Build