I suspect it's a Belgian copy. If it's not marked as a Webley, and has non-british proof marks, it probably came off the Continent. There is also the posibility that it's a U.S. made gun, perhaps a Forehand & Wadsworth, or a U.S. Revolver.
It's probably chambered in .44 Bulldog, which is essentialy a shortened .44 special. It also might be chambered for .442 Revolver, an old British loading. British rims are very thin, like .030", as opposed to the American standard of about .060". Check how much clearance is behind the cylinder. .44 Bulldog can be made, but .442 Revolver involves turning down rims to manufacture. Either is strictly a handloading proposal.
Another possibility is .41 Rimfire, but that usually didn't get chambered in Bulldogs. Regardless, all of these are blackpowder numbers. This gun was probably built before 1900.
Additionaly, unless that gun is MINT, it's over-priced. Webleys can expect to bring over $150, but the off-brand guns usually go for around $100.