That question will get some people scratching their heads.
Anytime a company made a change to the original specification it resulted in a variation number being assigned to the design. I just read a book on the B-29 and there were more variants than I ever thought about. Change the radios, give it a new spec number. Remove the tail cannons, give it a new spec number.
This only applied to production aircraft, field mods were vast, though many of them did end up being employed at the factory...which did meant issuing a new spec number...which filtered to the ones in the field as well.
My guess to answer your question would be either something from the Germans.