My first post. Hello everyone.
Agreed with Luftwaffemesserschimitt, the small letter designation indicates engine type. "a" is for Jumo004 and "b" is for BMW 003. Only four Me 262 A-1b ever built. Wrk nr. 130184 and 170078 were engine test beds. Rutzsatze designations for R4M is "R7" (example...