Thank you for your insight. Very much appreciated. I have talked this over with my gunsmith and the Ithaca with both barrels is on its way to be inspected and repaired. More photos when it comes back.
The real puzzle is not the screw fix but why the gun engineers/makers at Ithaca would add a locking rib extension, creating the "Lewis" model, modifying the Crass model and in doing so specifying the locking extension and sighting rib as one piece. From my reading and understanding this most likely has the rib with its extension silver soldered on to the barrel assembly, a rather weak joint, rather than brazed soldered, a much stronger joint. Hence, both barrels suffering the same failure.