bonny,
The British used different thread forms and the dia./tpi is only a part of it. Some may be 60 deg. included angle like ours, some are 55 degrees Whitworth, some are flattened at the points and roots, and some pointed, then on some the tops and roots are rounded. If there are more than one lock screw, they are all likely the same. You may be able to get a better idea by checking an undamaged one. You may get more help from a model builders website. If you find the thread, it will likely be hard to duplicate the screw, because it is so short with the thread going to the head. If you use a lathe, I find it easier to turn the spindle by hand and watch under magnification( uncouple the motor and cutting a thousandths at a time, the spindle is easy to turn by hand. If you can find taps and a die, you can make a threaded rod and oversize round nut. Screw them together, heat the nut red hot and "forge weld" it to the threaded rod. Shorten to length, dress/size the nut, and cut a sacrificial slot. Screw it into the gun, mark the location of the final slot( to "clock" it), cut the slot and reduce head thickness to flush with the other screw and engrave to match. This gives a screw with a thread all the way to the head.
If you do it this way, you can recut the form to a known form in the action, With similar diameter and same pitch.
Mike
