Welcome to the camp ! :v
FWIW, I've never seen or heard of a Remington muzzleloading double shotgun with back action locks, which your gun has.
Remington did, however, once upon a time import muzzleloading double shotguns from Europe 1855-56 (England) - which carried the mandatory proofmarks required by law in Europe.
Do you have a pic of the "Remington" marking(s) ?
The imported Remington doubles were AFAIK marked "Imported by E. Remington & Sons, Ilion, N.Y." on the top rib.