It does look as if it's been put together from two or more guns. Not uncommon, apparently, during arsenal servicing.
Unfortunately, your gun is in pretty rough shape. It's missing the hammer, the barrel band has been removed, and the forearm has either been cut back or replaced entirely. It's also been "checkered" in a pretty crude fashion.
As is, it's not worth all that much.
Smith carbines were designed in the late 1850s, and probably wouldn't have seen much use except for the American Civil War. About 30,000 were made, and almost all were issued to cavalry units, with some going to artillery units.
They fired an outside primed cartridge that was unique because it was made of rubber. Later versions were also made of a combination of heavy paper and metal foil.
Here's a pretty good article on the gun and the guy behind it.
http://www.guns.com/2013/07/02/the-c...se-for-rubber/