If the gun had been ordered new with a flat rib, the corresponding area in line with the rib across the top of the standing breech would have been flat.
I could certainly speculate that the gun could have originally started life as a Damascus barrel 16-gauge and then was returned to Hunter Arms Co. to be rebarreled in fluid steel as a 20-gauge. I know of a very high grade Parker Bros. that started life as a Damascus barrel 0-frame 16-gauge and was later rebarreled in fluid steel as a 20-gauge, and still later, in the Remington era, got a set of fluid steel 28-gauge barrels. Lots of things could have happened to these old guns in the past 100 years or so.
Will be interesting to see this story unfold.
