Looks like a nice old gun. From what I hear bore sizes ran that gambit. 15 bore wasn't that uncommon....most I've saw and owned were stamped with a bore size.
I've often wondered how much platinum weight is in the vent plugs....might be worth more than some guns ?
I suspect the little choke you are measuring might have been caused by barrel honing who's to say if it was intentional choke or not.