500Nitro,
I'm going to go off track for a second with regard to the rubber between the ribs.
My Krieghoff has a "conventional" rib all the way down the barrels from the stock up to the last three inches from the crown. Then there is something else, lets call it rubber. It looks similar to the rib but it's not. The front sight is where the adjustment for regulating the barrels is made. Remember if the barrels were welded all the way up how would you adjust them? So my thoughts are, the barrels are actually not welded at the last three inches and you twist a wedge which moves one barrel or the other to change point of impact. This plastic or rubber piece cosmetically hides this.
I haven't taken my gun apart to see if this is it exactly, but I'm pretty sure I'm close in my train of thought.