My experience with a few dozen sxs's over the years is that English shotguns are hit and miss about whether they can be resurrected for use. Belgian shotguns tend to have drawn steel barrels with heavier wall thickness and are more likely to be "shooter prospects" The ELG in the oval is the belgian proof and under the breech plugs Belgians often stamped the bore in millimeters.
An easy way to check for pitting, and nearly all have at least some. Is to put a pin through a long dowel and drag the pin against the interior barrel walls. (Like the way the dentist uses a pick to check cavities. A little rough scratching feeling like a needle on an old phonograph record is to be expected, but actually catching and or hanging up are signs of problems.
Often old belgian shotguns need new tumblers in the locks because the half cock notches are gone.
Many of the mail order companies sold inexpensive brand new muzzle loading shotguns even to about 1910. (Sears for instance)
Also, hardware companies and mail order outfits often purchased shot guns and had their own names put on them. So it would not be unusual for a small regional hardware company to have their own brand name on the same shotgun sold by someone else under another name. Toward the end of the era for such sales, some unscrupulous dealers would have names put on the guns that were similar or the same to well named brands of firearms. I have seen a "Barker Bros" double.
Better English Shotguns had silver colored "blow out plugs" on the sides of the breech plugs. Some shotguns were Damascus and some were "London Fine Twist" a fine twist should be checked very carefully, although it is somewhat less likely to be compromised than a damascus barrel.
Some doubles were imported from France and Germany as well.
Many of the cheaper English doubles had stove pipes for barrels and aren't worth anything but wall hanging.