Hi Brandon, and welcome to the forum.
"S/42" "42" and "byf" were all government mandated
ordnance concealment codes for Mauser manufactured guns.
At specific times during WW-II the German government office responsible for such things changed codes.
After WW-I, the Versailles treaty only allowed rework or manufacture of Lugers by Simson & Co. of Suhl Germany. There is speculation that the inclusion of "S" in the earlier Mauser code was intended to encourage the mis-identification of early Mauser guns as being made or reworked by Simson. The first Mauser military Lugers were marked with "K" (1934) and "G" (1935) date codes and a S/42 toggle. Early Krieghoff Lugers were also marked with "S".
By 1940, the Germans probably no longer saw any need to include the "S"...
Marc
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