It's my understanding that it is legal Federally to transfer a C&R firearm without a serial number. I see them for sale frequently on GB. I have not purchased one yet, but it wasn't for lack of trying. I was chasing a un-numbered CZ27 (not uncommon) not too long ago. I didn't win them, but was comfortable that it was legal. I have a C&R, but living in CA, need to also work with an FFL01. My FFL01 was also comfortable with it legally. Un-numbered C&R WWII era handguns are not uncommon.