[cheapthrillz]

Well l will try to help but without photos its just a guess. I have collected and shot model 12's for far more years than I care to admit. First, if the barrel has a choke marking (full, mod, imp cyl) then the polychoke is an aftermarket and greatly reduces the value. The rib is unventalated? But if it does have a rib then it is what is called a solid rib and is a bonus. Lastly cleaning metal parts with 000 steelwool soaked with WD-40, done lightly, is the standard way to remove surface rust. As to the checkering I would have to view it to determine if it is factory or aftermarket.

The barrel is marked as a Full choke. It has no rib. Here are some pics: