I looked at that seller's site last year. I think they know much more about handguns and AR-type rifles and just 'wing-it' on the older models. Probably use AI to get their questions answered.

This is the main reason I only buy shotguns from the people on this website , or their associates.
Tim

What does the Serialization book state about 132783? With a 1-frame, what gauge did it start life as? The K in the barrel steel mark location on the barrel flat is consistent with Titanic Steel in that era.

It is not in the book. But being a 1 frame the safest assumption would be 12 of course. Maybe 16.