Re: Mauser 1914 question
I used to tap some of those out for the M1911 grip screws, which aren't a good head fit, but are not too bad if you thin the head a bit. But the M1911 is an odd thread itself, being .150x50. I had the tap at that time, but it is not worth $35 to buy one for one job.
The trouble is that the combination of a small screw major diameter and a large head just is not common, except in grip screws. If you have a means of reshaping a head, you could tap for something common like 8x32, then shape the head of a common screw or hex head and cut the slot. Not hard to do, just takes a little practice.
Let me think a bit on that one and will try to get back to you.