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.
Yep it is deffinatley a 682 Unsingle they are a little bit heavier than my 680 Unsingle I guess it must be the added weight of the rib but this is the type I would want because if you got a regular or high comb and you might have to sand it down some for the flat rib barrel like mine.
Ok notice the difference in the rib metal above the wood in my gun picture and Pat's on mine it is almost level with the wood and on the 682 barrel it has the 1/4" above the wood section so this is a way to look at them and tell if it is from a 680 or 682 and above gun.