Help me with a Fidelity Research FR64 part

I got a mint FR64 tonearm at a great price. Sadly there was a catch it did not have the nut that fastens the arm at the base. It is a really fine thread (25 threads per inch) in fact my machinist doesn't have tooling for this.

so I want to know if anyone out there knows where I can get the said nut, or if they have one they can sell me - thanks