Help me with a Fidelity Research FR64 part

It’s likely to be metric.
You could by a thread gauge and possibly luck out with a standard metric pitch. A caliper will give you the diameter.

If a machinist cannot recognize the thread within 20 seconds run away from him, he is no good! It’s not a rocket science, I extracted the arm from the alu threaded male collar, brought it to a machinist and have it redone in stainless steel with a massive brass nut.

I'm afraid that the FR64 has a significantly finer thread than a standard metric/imperial machined thread - that is why I started the post...

Inapt. The word you were looking for viridian is inapt: not suitable or appropriate. Inept means clumsy. As in your command of language is inept. Like that.

Tacoma Screw, if you happened to have grown up in Tacoma, is a pretty darn apt name- both suitable and appropriate.

parrotbee if your machinist can't tap 25 threads per inch find yourself another machinist. Not that you will need one. Nobody machines a nut like this anyway. They are all standard parts. You just need to match up your threads and diameter with the type of nut you want to use.
https://www.boltdepot.com/fastener-information/Nuts-Washers/Default.aspx

Or you could just take the whole thing to Tacoma Screw. I bet they have what you need just sitting there and for a buck fifty, if that.