Proof marks will tell the story. Belgian will be a combination of different markings, along with "ELG".

Photos will be necessary absent of a hands-on inspection. You fail to share whether a muzzle loader or a breach loader. Hammered or hammerless? Gauge? Condition? Grade? The list goes on.

I would recommend against an antiques dealer as they seldom know the value of an antique gun. Value might better be determined by someone familiar with antique firearms with no personal interest in the acquisition of the piece.

B.

[Powder, Patch and Ball... Pennsylvania Rifles best them all.]