3/8 by 10 pin

[DeeDeeCues]

If there is a Harbor Freight near you, they have a digital caliper which is very inexpensive.

If you buy one you can measure the major and minor thread of your pins and see exactly what is going on.
Granted there might be a wee bit off but you’ll have a clear picture as to the differing sizes.

I have had one for fifteen years and it works great.
Just a thought!

Directly and accurately measuring minor diameter of threads with only calipers is not possible. By definition, the calipers will be angled to get in the root of the threads, and the thread is likely sharper than the caliper. A three-wire measurement and calculation is required.