Looks to me like somebody had their gunsmith make them a "Quail's Fargo Special". Factories seldom ship a Cyl/Cyl gun unless they are making a double riot gun (or "coach gun" as they are now called) for the CAS guys. I don't think SKB ever did that.

But I don't knock cylinder bore; it's more useful than generally assumed. I have two words for this gun: "ruffed" and "grouse."