In theory all of the BC-221 are functional equivalet. The different series numbers ( I guess that is what they are) indicate different production contracts. The BC-221-A ,BC-221-B and BC-221-D use the old six pin tubes 6A7, 77 and 76 Some use loktal metal tubes, 7B8-LM Most use octal metal, 6K8, 6SJ7 etc. There is an Army manual that lists different numbers