I would think it's much older than Civil War. Looks to me like it could be off a "Kentucky long rifle." That would make it date to possibly the 1700's. There is no cut out for a patch box, and most Kentucky long rifles had patch box in the stock. So it's also possible it's off a half stock cap lock rifle, that had a cap box rather than a patch box. If so, percussion caps were invented in the late 1830's, so odds are it dates before the Civil War. Here are some pictures that I'm talking about.
This first one it a Kentucky long rifle. The patch box is all that brass on the butt stock.
Close up of the patch box on a flint lock. There were long rifles converted to cap lock, that naturally would have a patch box, and there were also cap lock long rifles built as cap locks. So I'm not saying everything is the way I'm describing, just trying to pin down your find a bit, using general information, there are always those things that were different. Every gun was hand made, no two were exactly alike, there were no interchangeable parts. Eli Whitney came up with the idea of interchangeable parts which is a story in itself.
Here is a picture of a patch box opened. If you look closely you can see the cut out in the butt plate for the patch box lid to fit in. That's what's missing on your find. Doesn't mean it's absolutely, positively not off a long rifle, just odds are it's not.
Just to show you the difference in what I'm talking about, this is a picture of a cap box, so no cut out in the butt plate. And the last picture is a cap box opened with a cleaning jag inside.
