The grip screws use a "Whitworth" British thread profile. it changes the angle of the thread.
https://www.boltscience.com/pages/screw4.htm
https://www.practicalmachinist.com/f...screws.221376/
You will find the taps listed in a google search. Sometimes the taps are on Ebay.
Here is the fabrication specification for the screws (courtesy of John S.'s blueprint files):
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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