You should be able to call Spencer Rifles & find out what it was used for. Spencer Rifles: 615-325-3551 If they can't answer your questions, you should be able to go to a major public library and look up Civil War Weapons and find out where it came from. If it has a serial number on it (doubtful), Spencer may be able to figure out where it was made and where it was sent to be used in the war. You may have to pay them, as I doubt they have it in a computer database; it is probably on a hand-log somewhere, written long ago.
As for the "Merril" rifle, these were made in Baltimore between 1861 and 1865. They were short-lived in the war, though used by enthusiasts who liked em, but were bought by and used a lot by the railroad.
You may want to visit the following sites to do some additional identification:
Various Civil War Weapons: (Spencer Rifle is about halfway down the list)
Shooting Merrill's Carbine
Review of the Spencer Rifle:
Civil War Guns: Spencer Repeating Carbine
Various Civil War weapons: (Shows Merrill Carbine)
Guide to Identify US Military Percussion Carbines of the Civil War
Here is a web-sale of a Merrill Rifle: (!!!!, that's not cheap)
Merrill Mississippi Rifle
Here is a detailed review/description of the Merrill Carbine:
Shooting Merrill's Carbine