Wadkin ags 10 Lock Nut

[Amplidyne]

I agree square threads are a PITA. I did one on an eight inch machine vice a while ago and ended up making my own tool, based on careful measurement of the male thread.
Worth it to me as I picked up the vice for very little and it was in good condition apart from the nut being worn out.
But if you just want a working machine then much easier and cheaper to just replace it with something more widely available, if of course you have the means to do it. If you had to pay someone then the £90 nut might well be cheaper.
The joys of old machinery

Never actually cut one, but everything I've seen, both online and in Machinery's Handbook point to the tool having to be just right, and that the threads are a pain to cut.

 

[Fergie 307]

To be fair once I had made the tool actually cutting the thread was pretty straightforward.
Fine tuning the tool wasn't!
Very difficult to get the shape just right so it doesn't bind as you get further into the cut.
What I ended up with was something very like a parting off tool but shallower and with the section supporting the cutting face angled to follow just inside the cut thread, if that makes sense.
And you have to clear the chips constantly.
I ended up having the coolant firing straight down the bore.
I had a couple of test runs on aluminium before launching into a big lump of phosphor bronze.