Bill, the E above LG within a circle with a little crown at the top is the Belgium proof mark showing that the gun was made & proofed in Liege, Belgium.
Many English gunmakers would order barrels from Belgium for their high production models right up through the early cartridge guns of the early 20th century. Infact, many late 18th and early 19th century gun makers imported some of their guns from Belgium with company's name stamped on them. That is one of the reasons that Browning O/U's were made in Liege, Belgium for years. Turkey is now the cheapest place to have guns made for you, Kimber, Renington, S&W, CZ, and De Haan to name a few.
You can find these guns in about every gauge. And measuring the muzzle can not always be correct because of ranrod wear. My 1850's English double has Liege barrels and is really an 11 gauge and I have to order 11 gauge wads and caeds for it.
All the various wads, over powder and over shot cards are available for just about every gauge from Track of the Wolf also. I shoot patched .750 (3/4") round balls in my 11 ga. and my bore ga. shows the gun to be an 11 ga, at the muzzle. You'll get it figured out I'm sure.
Did you know Bill Wasson is an avid BP shot-gunner too?
Below is my 1850's 11 ga.
Dave