Raimey:
If you click on the first forum then go to page 2 there is a thread titled 1855 British bore size proof stamp. Birmingham proof 1887-1896 shows the same proofs as are on one of the flats of this gun. The serial number was added at this time I believe.On another flat is stamped F,E 430 which is original to the gun. I should also mention the frame is also Birmingham proofed.

Bob