Oh, there's a comma in there: 6,3mm..... Yes, 6,3X52R would be correct. Still, very, very odd that the chamber length isn't present.

F.K. could point to Franz König, maybe Fritz König. By the way, there were so many König mechanics in Zella - Mehlis that would couldn't stir them with a stick(probably a total of 100 or more over time). But any Franz K. or Fritz K., etc. in Zella-Mehils might be a match.

SS more than likely is Suhl barrel mechanic Stephan Schilling.

Serbus,

Raimey
rse