At the time, the Brits were ahead of Germany in product standardization. The Whitworth thread was a documented industrial standard and Ludwig Loewe adopted that standard not only for grip screws but for barrel threads as well. Loewe also looked to the USA for standards for mass production as was initiated early on by Eli Whitney. Loewe wanted to produce quality and interchangeability so he wasn't reluctant to stand on the shoulders of those who had already been there, done that.
Ron

As described in my thread on this subject some time back, both a 3/16"-30 and a 3/16"-32 tap would thread holes that the three threads of a Luger grip screw would comfortably fit in. :rolleyes:

http://forum.lugerforum.com/attachme...0&d=1302648889

http://forum.lugerforum.com/attachme...1&d=1302648889

The #10-32 would not go all the way in.

IMO having the correct size Whitworth tap is nice but not essential for only three threads. :)

From the Luger blueprints in TBLAP -

http://forum.lugerforum.com/attachme...0&d=1299622711

Yes, as a matter of fact!

Such things interest me.

I used this method for a buggered-up/cross threaded Luger grip screw hole. The tap thread was short enough to 'free-bore' once it cleared the 'good' threaded hole. I also used an old worn/dull 3/16"-30 tap as I wanted to push the metal back in place rather than cut it. Used oil, came out fine. :thumbup:

I made a pair of grip screws. They are 0.010" small than the 5mm screw, but 0.8mm pitch. I used a 60 deg screw cutting tool.
Not Whitworth as they are too course.
I'll about to make some more with over size heads so as to stop the grip movement. Standard ones are 0.420" diameter

I have made grips screw by screw cutting the lathe. At the time, I found them to be 0.010" under 5mm and 0.8mm pitch. Just checked my taping chart and they are 3/16 x 32 tpi or 3/16 BSF.
Yes, I know, what are the very clever Germans using a Englander thread. That I don't know??
Maybe a Reasonable Knownable Person can answer??

The best metal fabrication and manufacturing machines made at the time it was initially designed and produced were made in England... does that answer your question Mark? :)

IF YOU READ the Practical Machinist thread noted above, it explains why Germany would use a British thread on their screws. It is a GREAT read!