I'm building my first form 1 suppressor, and want to go to with 1.375x24 (Hub) threads. I'm finding conflicting info on the major and minor diameters. One source is showing 1.370 major and 1.335 minor, theoretical machinist is showing 1.372 major, but 1.319 minor. The major diameters are close enough, but the minors are .016 different...
Any input on which is accurate?

I would look up the spec in Machinist Handbook and use that. Or, buy your mount and use it as the gauge. It’s probably the safest bet.

Yes i would use the adapter/mount as a guide. You have no idea what the thread spec is on the mount. If you not making things to sell then its better to have a good fit rather than meeting a thread spec

If you Haven't found your answer here it is.
1-3/8 x 24 UNS 2A male thread
Major diameter: 1.3737 max / 1.3665 min
Pitch diameter: 1.3466 max/ 1.3422 min
Minor diameter: 1.3241 max / 1.3123 min

1-3/8 x 24 UNS 2B female Thread
Major diameter: 1.3867 max / 1.3750 min
Pitch diameter: 1.3536 max / 1.3479 min
Minor diameter: 1.3396 max/ 1.3299 min