Ithaca:

The British proof marks in use in the early 1870s differ from those in use 1875-1887, which in turn differ from those of 1887-1904. It's usually a simple matter to date a British gun to one of those periods by the proof marks, but "The proof marks are Birmingham" is of no help. We need to know what marks are present, preferably photographs.

"Serious rifles have two barrels, everything else just burns gunpowder."