[DonVoigt]

The "m" does not mean Mauser, it is a part of the serial number.
Lugers were numbered in series by year and started with 1-10000 no suffix, then an "a" suffix,
then "b", etc. Not an absolute but accurate for the most part.

The S/42 code is or was on top of the middle toggle link.

Your side plate, L piece, trigger, and sear bar and plunger all must work together and be fitted to each other and to the frame to work correctly. The plunger and spring would be a likely suspect, but not the only one.

You might get farther along more quickly by buying a new sear bar- if rusted/stuck the tiny spring is probably Kaput. The spring is near impossible to find by itself.