There are a lot of "odd" thread sizes in guns too. 7/23 x 40 and 9/32 x 40 are common peep sight sizes. The grip bushings on a 1911 are small very fine threads and not a common fraction size. Something like .234 x 60tpi if I remember correctly, I'm still waiting for the full effect of this mornings coffee. I repair a lot of old guns and some funny old threads pop up from time to time. Sharps rifles use 10-28. Not the standard 10-24 or 10-32. Split the difference at 28. When did the modern standards come into common use? After 1850's I'm guessing.
Many supply sources for the odd taps and dies if you look under "Special Taps & Dies" or some variation of that in the catalogs. I've build a fair selection of them but by no means have I got them all. I even have a #7 machine screw tap and die I found at an estate sale. It just looked odd and like attracts like, not so with magnets. Machine screws go from #0 to #14. And a plethora of pitches......it's fun to design things that use the odd sizes and let someone figure out what you used, 'cause it sure ain't standard!
