I have a Lewis 12 and a Lewis 16 and neither has the rib problem you show. I can only guess as to what made the rib separate on both sets of barrels. The only thing they have in common is the receiver they attach to . Maybe its a receiver problem? If those screws were down the bore a few more inches I would worry about their cutting into the barrel wall, but where they are the walls are about 150 thousandths so probably not a factor. Field grade Lewis models rarely pay for themselves except in the enjoyment of shooting. Best.
