Here's a pix of the screw with the new left hand 7 TPI stub ACME thread. As you can see it would be a bit of work to completely machine one from scratch including the collar, hex for the handle, and thread for the retaining ring. It was a lot easier to just turn off the worn out ACME thread, build up the shaft with my MIG welder and turn the left hand 7 TPI ACME thread. I welded it pretty hot using the the MillerMatic 252 settings for 1/4' steel so I think I got pretty good penetration on the base of the shaft. I used a block of copper clamped on the end so the welding didn't destroy the center hole on the end of the shaft, which I used to turn the new thread.
A new screw from KURT is $267.00, and that's not in the budget right now.
I just wondered about trying to harden the new threads since the original thread was worn so badly. Curiously the nut didn't appear worn at all, and the new thread fits the nut quite snugly?
I'm waiting for the D-60 repair kit and speed handle to get here from ENCO, then I'll put it back together. Trying to decide whether to paint it again, almost all the original KURT blue is gone except for in a few places. I removed the AL name plate which was completely obliterated except for the stamped serial no. Obviously the vise has been well used (misused), surprisingly the rest of the vise was in pretty good shape, and the rotary base looks almost new except the paint is all worn off, probably a testament to how well these old vises were made.
I appreciate everyone's suggestions and help, this is a great forum!