I don't know a great deal about them but I do know that if that is the real thing then it is in AMAZING condition for it's age for what I've seen in the past. A-****ing-mazing. I do know that the a lot of them had an 18mm bore diameter. Some where near the butt on the right side (could be wrong, may be left side. Going by an old scotch drounded memory) it should have a stamp reading "Felix Escoffier" and the year made. I'm thinking they stopped making them in 1873 or 1874; Thinking 73.
As for the value, I dunno. But I would be willing to take it off your hands; real deal or not. That thing's just too big and will take up far too much space. You don't need that around your home and I care enough to help you out on this one.