There has to be a manufacturers name and model number along with a gauge stamped on to the barrel. It doesn’t look like a rare or even all that old of a gun. My guess is the it is just a 20ga single shot worth about $100. It goes bang and is very simple.
It looks middle eastern to me. I saw pistols similarly decorated like that at the market in Tunis back in the seventies. That would also explain why there is no manufacturer stamped on it. That's just a wild guess though.
I`ll disagree with the $100 I have no clue as to value 20 gauge single shot engraved I`d opt for an appraisal from a certified individual...However if no other maker mark ie browning/benelli/savage can be found I side with a Turkish import eh then $150 would be max/pushing (NF) to New England Firearms? Them little handy-rifles/shots are climbing as like Jennings Raven Boa Jimenez a $45 to $70 pistol all day long TODAY though Seen $100 to $140 I was like HEY then was told no longer made fools driving prices insanely so I learned Value and True Value. True Value is what it is actually worth. Value is what a body will PAY for it.