My first thought is that it is American, 1860-80...no way to ID if there is no name anywhere (many American gallery air rifles are unmarked). That said, there are a few unusual details that make me question its actual origin:
1) exposed axial barrel pivot-mostly only seen on German pieces, but not unheard of on American specimens. American makers tend to hide that mechanism in the forearm wood.
2) double set triggers-not unheard of on American pieces, but usually only encountered on very high end pieces.
3) barrel latch on a swivel breech...first time I've seen that (nicely made American pieces will sometimes be precision fitted with a "cradle" on the receiver to lock the barrel tight)
4) crank boss looks to be made of flat German silver material-most gallery guns use a cast brass or machine steel boss to support the stress of the crank on the rack.
5) Octagonal to round barrel-certainly European in flavor, though I've got a couple of American pieces with this feature.
All in all, a nice gallery airgun with some unique features, though it'll be almost impossible to definitely point to a maker without some kind of markings. That hurts the value somewhat-collectors LOVE to know who made their stuff!