Are there any other marks on it? The Brits were fond of putting proofs all over everything. The Martini Henry's were mostly 19th century guns. If it is an antique it is much simpler to sell since you can ship without an FFL.

It is very nice looking and looks like a more modern caliber than the 577/450. If it isn't marked it would be a good idea to do a cerrosafe chamber cast to find out what it is. I did that for a rolling block once.

Steve