@ RwBev
I tested your theory by gently running a knife along a small section of the rib. I only did about a half an inch worth, as I don't want to mark up the gun. The result was a strip of silver where the knife had made its course. Your theories relevance can't really be put to the test with this particular gun, due to the proof marks being so difficult to read. But from what I can make out, they do compare the best to some of the Birmingham marks, which does help with your theory, as those are English marks.