Belgian origins I'm pretty sure. Nagant family of double actions. As to specifics... not up to speed nor do I have me books at work to look up more info. Just going from memory.
The cylinder arbor (the part the cylinder rotates on) seems to be either missing or broken off.
The gun is a general type of European revolver of the c. 1870-1900 era. I am pretty sure it was made in Belgium and if you look closely at the back of the cylinder you might find an oval with the letters E L G, the Liege proofmark.
Those guns are fairly common, even in the U.S. and are of little value, perhaps $200 tops if in excellent condition and working properly (which they rarely are).
A nameless open top centerfire, one evolutionary step beyond the pinfires.
It is similar but not identical to a Gasser 1870 Austro-Hungarian infantry officer's revolver.
Maybe a Belgian or if not proofmarked, Spanish copy of the Gasser.