If you have a good condition nut (like the one on the other spindle, maybe?) a good machinist can duplicate it. That might be an e$$$pensive approach, but it's a "last resort" possibility. That thread diameter and pitch is a real oddball- - - -no wonder they're scarce!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!