I'm always curious b/c at some point the czech makers stopped serializing certain parts. You sometimes see "1" or "4" proofed parts supplied by Mauser with no s/n, and sometimes on very late czech 98's you see milled bottom metal with a lot of machining marks left (not polished) and no serial number. Usually they have a "d" proof where the serial number would go. Supposedly these were made b/c Mauser stopped shipping stamped bottom metal to Czechoslovakia at some point, probably when allied air power made materiel difficult.
As an aside, I have an RC BNZ43 with a very late war czech made bottom metal on it.