Pat:
By the description of it having the 1/4" step in the rib then it seems that it came off a 682 or later version of gun which I think would be more desirable.
In the picture above is a flat rib unsingle that came on my 680 and none of the first 680's had ther step in the rib like yours and a lot of times people would have to shave the comb to line up on the beads because the high rib only took place of the top barrel only.
Even a 870 remington had a higher rib than my 680.
It sounds like the price is very resonable if I didn't already have 3 barrels for it already I might be a buyer.
I have a 34" top single barrel that came off a 682 and just wanted to say that I have had a couple more 682 barrels that I have traded off in the past and they would all fit my gun with no problems I think I did have to file a little on the end of the ejector's to make it perfect in ejecting the hulls on 1 set of barrels.
Foghorn220