Update:
By way of confirmation from Graham Greener (consistent with the barrel markings) the gun originally left the manufacturer as a 12 bore and was made in 1879.
I took it to Bill Schwarz in Ellijay, GA yesterday and he measured the chambers and wall thickness. The chambers are now 2 7/8 and 10 gauge shell dimensions. Min wall thickness left barrel 30 thou, right barrel 35 thou. There are no other visible issues, faults with the barrels, receiver, action, etc. Bores are mirror bright. The gun is on face, but has a slight bit of play between the barrels and the action. He said the steel looked to be very high grade Damascus/Laminated Steel and in excellent condition. He suggested it would be safe to shoot with low pressure 10 gauge loads.
It is hard to believe the gun left Greener with that much material in the barrels, but I am quite grateful that it did