This may have been a rifle converted to carbine length, either after the Civil War or by the owner following the War. So far as tracing the piece is concerned, I think I would start by determining who owned the house "way back when". County tax records might be a place to start. Ownership going back to 1861 would be a starting point. The owner or a son might have served in one of the Michigan brigades...maybe, even with Custer! I would trace the ownership of the house for at least 20 years after the War. If you can find the name of the owners of the house, you may be able to get records of service and pension records from the National Archives Navy & Old Army muster records. Also, the Michigan historical society or the county society may have more data. The fact that the Spencer was found in the attic, rather than at a gun show, makes it more likely that the owner of the house was associated with the gun. It may take awhile, but can be like putting together a puzzle or solving a mystery (which it is). Good luck.