Castle nuts are intended to be retained by cotter pins, so why would you need reverse (left-hand) threads?? :whazzup:
Maybe General Thread in SE PDX, or you might have to have them made by a machine shop. :shrug:
They are for a drive assembly on an old MF tractor mower. The cotter pins act as a shear pin too, so if the blades hit something very hard, the pins shear off, and the nut can spin rather than breaking the driveline or blade. They are not torqued so the cotter pins keep them stationary as well.
Tough to find even in tractor/mower retail industry. I was hoping maybe theres something in the boat line of products similar to what I need. They are the same as a prop retainer nut, but reversed SAE threads.