Keith,
The enclosed quote was taken from an much earlier post by Denny Graham. Maybe it will shed some light on your problem.
"Again like Ken mentioned about the thread it is a special thread, actually it is a NS (National Special) 5/8”-20 thread. McMaster pn. Is 26005A148 and it will run ya $66.89. This wouldn’t be a waste if you were gonna work on a lot of these because the thread is the same from 1929 all the way up to 1954 (as far as my references go)."
Stuart
