Daryl,

I am positive it is a 10 bore. 10 gauge snaps caps drop into the chambers, and the muzzle end of the bore measures .758". If my math is correct, that should be pretty close to a Modified choke, right? The "12" markings on the barrels threw me as well. The markings you see above are the only markings on the underside of the barrel, with these being the only markings on top: