Nellis & Sheath.
It is most likely Hollis & Sheath, an English gunmaking firm. Many of their guns were imported into the USA in the 1800's.
Hollis & Sheath, Birmingham, 1849-1861....gun and pistol makers.
The company was reorganized in 1861 as Isaac Hollis & Sons.
I plan on cleaning up and refinishing the rifle for a display case but some history would be nice.
You said it's a double barrel, so it is probably a shotgun, not a rifle. Also, do not fool with it as you will destroy any intrinsic value it may have. IF on the chance it is a double rifle, that is even more reason to leave it alone. Amateur refinishings will immediately reduce its value by 75% or more.
