It could definitely be rare, but that doesn't always mean that it's valuable. The first thing I'd want to know is what sort of ignition system and calibre it is, and who made it and where. Those are the sort of things that's going to take a close look to determine, and maybe even taking it out of the wood. I'd also want to make sure that it isn't loaded now, given the unusual method of loading. It looks like there's an internal hammer operated by the slider on the tang. The hole is part of the loading tappet, and is basically a ball valve that spins open and closed by twisting the wing nut on the right side, so something can be placed in a hole through that valve, then it can be spun back into line with the bore (but we don't know yet if that something is supposed to be a propelling charge, primer, or projectile). If you can take a lightly-oiled cloth and wipe down the outside of the metal parts, you may be able to see some marks or writing somewhere that might help to identify it.