Muleskinner was correct ...but he wasn't quite specific enough. It's not from a typical civil war socket-bayonet. I'm quite sure the object is the broken-off top of the barrel-ring on a US Model-1855 Saber-Bayonet.
Examine the photo at the end of this post. That particular model of saber-bayonet (there are many other models) had what collectors call a "scroll" on the top of its brass barrel-ring ..and therefore is referred to as a "scroll-top" hilt. It is not a common feature on saber-bayonets, so it is one of the identifying characteristics for the US Model-1855 Saber Bayonet. It was issued with the US Model-1855 "Harpers Ferry" rifle, and also with some "Mississippi" rifles. You can read about it here: http://www.collegehillarsenal.com/shop/product.php?productid=711
exact down to the angled cuts on the round "scroll" thingey. Next mission, should you accept it, is to scour all museums with this type of bayonet to find one sans scroll. Now, wouldn't that be an amazing feat!