The reason you have not seen one before, is because Winchester did not manufacture it. Somebody with a wild imagination had it made. If you look at it closely, the rib is a three-piece amalgamation. The center piece require milling out a section of the barrel ( thereby eliminating the original rear sight dovetail slot) and then screwing it to the barrel. The offset proof mark is simple… there are a lot of proof mark stamps in the hands of people who specialize in making up questionable Winchesters. If someone had actually ordered a solid rib installed on a Model 94 Carbine, Winchester would not have milled a rear sight dovetail slot, and the rib would have been a solid piece from end-to-end.

Bert