The rifle is very old, indeed, indicated by the wedge holding the forestock, and the fact that the rifle has Damascus barrels, and is a hemmer back action, Jones under lever action. I doubt there will be much in the way of proof marks on the flats. The rifle, if 450/400, is most likely the 450/400 3 /14" Black powder version, and would have been made in about the late 1880s for the coiled brass cartridge. The rifle, from what I can see seems to be in fairly good shape. I have a 20ga/58 Berdan Carbine cape gun that is almost identical to this rifle, but was made in KASSEL,(old spelling) Germany. This rifle should be a no paper work rifle, as it was certainly made before 1898, and factory black powder ammo is no longer available for it.
I would reccomend a chamber cast, and a bore slug, for sure! If everything is as good as it seems to be, this could be a very nice rifle to have, at the right price. The price is dependant on how the cast and slug turns out!