Often, those triangular fillers between the barrels and ribs are referred to as keels. As gunman says, sometimes they are actually part of the rib. More often, they are made as he describes. Typically, they are short enough that cutting the barrels even an inch will totally remove them and leave an hourglass shaped gap. The gap is often filled with solder, and the gap between the barrels is a clue that the barrels are not original length since the barrels are usually touching at the muzzles on a shotgun. Really crude gun butchers either leave the gap open, or fill it with epoxy.
