A 506 is Columbian, so I would doubt it being whitworth. Check the bolt for an unseen burr or upset thread.
Good young eyes, a bright light, and a precise ruler will work. Plus magnification for me, as I'm missing that first item.
I'm missing the first item also. Outlaw was right. I took a points file, and a triangle file and cleaned up the threads. They got pretty bunged up when I had to chisel the swivel handle off. I was able to salvage it from that, and after putting some bright light on it, donning my strong reading glasses, and taking about 15 minutes to carefully clean the threads to the point that a new 1/2" x 13tpi nut goes right on. I'll buy an acorn nut if I can't find a replacement handle for that swivel.
Here's a before and after so far. I've got it cleaned, and stripped the coats of paint and primer off with my wire wheels. The black was mostly grime I guess because the paint underneath it was definitely blue, on top of primer and mostly stripped original red. Still got some work to do, and this one will get painted when the weather warms up.
I could not get the jaw pads off, and in fact broke the standard bit for my impact driver trying.



















