They made the original screw and other parts from wrought iron,which has many layers of silica in it. The parts were then case hardened. This seems to "consolidate" the metal. But,try to CUT the threads into the SOFT,unhardened metal, and pieces of the threads will fall off. The threads will have to be CRUSHED into form by screw plates. These have progressively smaller holes in them with the same thread. You start with the largest hole,which presses a bit of thread onto your rod-to-be-threaded. Then you go to the next and the next.
I made a screw plate for the gunsmith's shop in Williamsburg,being the tool maker.
The taps were sold as SQUARE taps with the corners threaded. They work great,and I have made them for myself,also triangular,when I need a quick tap to fit an antique mechanism I am making missing parts for. I have a bunch of oddball LEFT hand taps that were only ever used once,as threads were not standardized yet.
The thread form I used on the jaw screw of this pistol was turned on a lathe,but I was careful to use an authentic thread form. More like a "tall light bulb thread" than anything I can describe. A tall "knuckle" thread,in other words. Very rounded roots and crests. And,not 60º threads,either.