Without seeing the gun, the following would be my speculation: If the frame is stamped "O", the gun is certainly a Grade O gun and was most likely originally shipped with Damascus barrels. At some point in the gun's later history, the original Damascus barrels were damaged; or the owner became concerned about the safety of his Damascus tubes and sent the gun back to Hunter for a new set. I have seen a number of Smith guns with factory replacement barrels (or with a later added set) that were of a lower grade than are cataloged for a certain model and so marked. These guns will have all correct factory serial number stamps; but with lower grade barrel steel type stamps/trademarks. But from my observations it seems that in most cases the customer specified an alternative/replacement set of Armor Steel barrels; perhaps this was because that choice would be the least expensive way to go, but Hunter's Armor Steel barrels also enjoyed (and still do today) a great reputation for strength and shooting qualities and represented an excellent choice and buy.