German military numbering always began at 1 (no leading zeros), then to 9999, then to 1a to 9999a, 1b to 9999b and so on for each manufacturer in each year.
So there would be a fair number of three digit numbers, even if those with letter suffixes are not included.
But there may be a problem with that gun. S/42 is a WWII era code for Mauser, starting in 1934, but the Germans did not use unit markings in WWII, and by 1934 there was no separate Bavarian army. Maybe I am missing something but could the gun be mismatched?
Pictures would help.