I'm reviving this old thread, and bringing it back on track. The topic is primarily Yugoslav markings as indication of whether a part is German MG42 or Yugo M53 SARAC.
Having handled a number of MG42 and M53 kits, including excellent condition complete (including receiver parts) kits, matching numbers kits, MG42 kits w/o any Yugo markings, destroyed post sample kits, and more, here are my conclusions.
1. Yugos used German parts. A number of M53 kits included German made parts and assemblies. The recuperator assembly is a commonly found German (bpr) item found even in Yugo-make M53 kits. I will say that "I" have never encountered a German rear sight assembly or grip stick on a Yugo-made M53, but virtually every other part/ assembly I have.
2. Yugos used German receivers too. There is actually a good thread on this on a European forum, with some claiming co-marked (eg. marked "MG42" etc as well as "BK") were forgeries, and others rightfully (IMHO) citing it would make no sense to do so. The latter I believe is correct. I have a well marked 1943 BNZ with WaA that has been stamped with the ubiquitious "BK".
3. "BK" is an acceptance stamp, not an indicator of Yugo production. Based on the above, and other research/ discussions on other forums, it's clear that not all BK marked parts were made in Yugoslavia, but were in fact wartime German production. Thus BK is definitely not an indicator of where the part was made; thus I presume it is an acceptance, test, or property stamp.
One thing remaining to study is a "feeling" I have: that some late war German MG42s may not have the AA sight base installed. But that is a different subject and one the hive must dig into by examining specimens in greater detail.