if the Winchester proof is on the top of the barrel and covered by the rib it is likely an aftermarket - Simmons being among the best and most common choices

if the proof mark is off center and still shows, it was factory done (still could be a Simmons, but done at the factory.

is it a 3"?

I’ve not had in hand yet . I’m Assuming Simmons is on the side the rib but could be wrong . And yes it’s a three inch .

A friend(?) of mine had a Winchester proof stamp. Any vent rib gun could be turned into factory with a touch of the hammer

I also read somewhere that most of what we’re called factory vent ribs were Simmons BUT the ones that were considered factory DID NOT have Simmons stamped on the side of the rib .

maybe - but there would be two proofs unless they went to the trouble of removing the rib, striking the barrel to remove the original mark and then rebluing

a lot of work

Tell us exactly what you have, and its markings, when you get it in hand, and we can give you a better idea of what you have. You can tell us about the proof marks, but pictures of the rib are necessary to determine originality.

Gunbroker pics of my less than exemplary specimen !

I had a Model 12 Heavy Duck two barrel set at one time with a 32 inch and a 26 inch barrels.

The 26" set were factory original uncut with 3" chamber ?

Yup, it was all straight as a string, 26 inch barrel was marked Imp Cyl and all the serial numbers matched.