This is one of those finds that makes you wonder how it ended up where it did. By the war that pistol was long obsolete, and I'm sure no one on the Northern side would have brought it down with them. Was it captured from a rebel soldier? Taken from a home? If only some of these things could talk.
However it got there, with a careful cleaning and preservation it'll look great on the wall. You definitely want to get that rust off and get the inside of the barrel cleaned out and then hot wax it before it starts coming apart.