Hey Deanno,
Here's one of my trap-shooting buddies from Ft. Dodge. This guy owned a lot of "iron" and in his day produced more diesel smoke than Cummins. Notice anything unusual on the breech end of that rib? My guess is this was an after-market piece designed in the basement by some tweaker/tinkerer much like the "K-Man" himself. I agree with Brian that it's a false rib, emitting a similar sight picture as a full vent version.
