Not recessed, jug or anything else similar, in fact the opposite of such. This choke is a constriction on the tube albeit with a flared muzzle. Most likely it is just a normal choke which has been messed with over 130 years.
I had one like this quite recently and when I mentioned it to my barrel man he had an immediate and obvious reason for it...but I forget what he said! Age is a terrible thing!