barrel/choke
Just for grins, I'd be curious to know how the gun is choked. If the barrel is indeed 26" from breech face to muzzle, I'd be surprised it is FULL choke as somebody has suggested. On older guns like that, a true 26" bbl was often IC. Close the action, drop a dowel down the barrel till it contacts the breech face and mark the dowel, then measure. Factory lengths should be 26, 28 and30". I'm thinking that 26" FULL choke barrels came along much later. I bought a Rem 870 26" FULL 12ga 3" mag barrel in 1980. It was the first I'd ever heard of in that configuration from any maker, and I had to that one direct from Remington. A 28" could be either FULL or MOD, and a 30" should be FULL.
It could be that somebody lopped a few inches off a longer barrel and it is marked FULL, but actually now has NO choke and is actually CYLINDER. Since the gun has been modified to mount a scope, it makes me think it could have been shortened to make a slug gun.