Rick: To answer one query, the tang screw is indeed a screw; the threads are so faint it's not at all easy to make them out, but there they certainly are. I suspect it's just become so worn that someone's peened over the end and left it at that. In fairness to whoever it was, if the bloody barrel's working up enough kinetic energy during recoil to put any real stress on the tang screw, you're doing something wrong in any case!
Edit: It seems that (according to a very helpful chap on another forum), this is an EIC Baker pattern lock, made from 1818-1839. It also seems, annoyingly, that the cock was originally secured by something called a "bun nut" (found a few pictures but can't find a Google match for the term itself) secured to a threaded extension to the square, and he suspects that this extension has been peened over during the conversion as it was beyond the capabilities of the local craftsmen to make a replacement bun nut for the new hammer.