I'm not even remotely a Mauser expert and I'll be interested to hear from those on the forum with actual knowledge. However, for what little its worth, I've read that Mauser stamped a lowercase "m" on the bottom of the receiver behind the recoil lug.

I've also heard opinions that the commercial actions didn't have the thumb cut out in the receiver wall or the stripper clip guide. However I don't believe this to be correct as I have seen Mauser advertising from the time in which not only do the actions have the thumb cut out, but also use the stripper clip feature as a desirable feature for the hunting rifle.