They've been stamping proof marks and other identifying marks for centuries.
I can't tell for sure but the forward mark on the breech looks somewhat like a crown over an "F" or a "B".
A Crown over F might indicate a "Foreigners'" mark of the London /gunmakers Company.
This mark has been in use sence 1741.
That doesn't signify that the gun was made in London, only that it was "Proofed" there.
Even today, guns not made and proofed in Great Briton require a proof test before they can be used there.
I don't see anything for a crown over B mark.
Of course, the mark may not be either of these as it is hard to see even after massaging the image with my Photoshop.