Mike,
Gunman is correct about the screw threads, they are most likely to be a BA (British Association) size. Most of the British Gunmakers used them, Charts of most of the British threads are available on the www including the tapping drill sizes.
B.A. Taps and Dies are commonly available here in Britain and most Model Engineering supply companies stock them. In the USA they would very likely be a custom made product, (expensive). You can also buy them off Ebay in you type in the correct search, and are prepared to wait until they come up for sale.
Harry
