Cadet it's a muzzleloader with re-case colored breech plugs ?
That's the key to the damage. I'm no gunsmith but I do talk to a really good one all the time. He told me that when you go removing breech plugs he builds wooden blocks to contour to the barrel...my best guess is the guy that removed the breech plugs during restoration crushed or smashed the rib. The rib was only smashed on the right side the breech plugs would screw out counter clockwise.
The barrel tubes on this gun inside and out were pistine. I couldn't spot any damage to the tubes with high magnification....I gave it alot of thought but I wondered if the damascus tubes could be damaged from the stress of mashing the rib on the right side.
