FFS, the drag marks on the cylinder are supposed to be there!
Um..... not on a single action they aren't. It is evidence that someone, likely more than once from how clear a line it is, pushed the hammer down from half #### and turned the cylinder against the bolt. Or that went past the half #### but not all the way to the full #### and rolled it around loading the gun. Both being No-No's in the single action world.
You're likely thinking of modern S&W's where the drag line IS normal and unavoidable due to how the timing works in double action.
At any rate the drag line is there and it's part of the history of the piece. I can't imagine any way to "fix" the line without ruining the lovely patina of wear that's on the rest of the cylinder and gun. But I'd love to be proven wrong on that count.
You're a very lucky camper to have such a nice example now. Be sure to treat it to some days at the range with a nice pillow to set it onto when not in your hand.