All 90 series had steel housings marked 0-1. IIRC there was some early ones marked S-F in stead of 0-1 at least on the HK41 and 43, but I can't remember if the firs 9x series had these.
91 had a long rear tang, 93 & 94 the same short rear tang housing.
Same innards except for caliber specific ejector, and 9mm hammer spring in 94 trigger mech.
Hk91 trigger housing
hk93 trigger housing