I guess the originality of the rib doesn't make much difference on a sleeved gun. My best guess is that the original rib was replaced . Possible reasons are the sleeved barrels are longer than the original, or that the original rib did not have the correct contours to fit the sleeved gun barrels. A check of the rib at the muzzle would shed some light. If the original integral keel is in place, that might answer the question.