To get a definitive answer, you might have to search back into the
archives
of SAE, ANSI, ASTM or Mil-Specs


Or Whitworth.
I thread standard, metric, BSF, BSP, BSC, and Whitworth (BSW) at work. That
makes some systems tricky to set up. I work on and make parts for vintage
Lotus race cars, along with a few similar marques.
I don't know why there are 3 sets of drills, but I won't be caught without
them. It's a lot easier to set up a bunch of 1/8 rivets in #30 holes that in
1/8holes.
--
Stupendous Man,
Defender of Freedom, Advocate of Liberty