Can't recall the exact pitch but I do know it is non standard. Does the right tumbler (stud) appear to be original? If so then you can measure its length, count the threads and with a little math should be able to determine the actual pitch. Coltparts.com has a tumbler nut listed. Maybe contacting them will provide an exact answer.

Hank

Yes, I did write a book. It's called "The Classic English Double Barrel"