Those screws were never standardized! Every set-trigger maker (yes, "Stechermacher" was a specialized profession in Suhl and Zella-Mehlis) used his own screws, some with metric sizes, most Whitworth threads, as did the Mauser, Oberndorf factory. Even pre- and postwar Mannlicher-Schoenauers differ, Steyr changed to metric threads only after WW2.