Those barrels look to have been un-breeched. The rib on the plug does not perfectly align on top and the keel piece, or short rib on the bottom looks to be loose(?)
Removing breech plugs is a major undertaking and they can be very difficult to realign, this also might account for jug choking that you are apparently measuring. Jug choking is only reliably done with the plugs removed.
Platinum vents were quite common throughout the percussion era, and when clear will puff smoke as the gun is fired. I'd think twice about shooting that one, and have an experience person do a very thorough inspection.
I used to shoot and work on muzzleloading guns quite extensively, my fav double was a 14 ga. Belgian gun. I still have a 20 ga. flint gun of my own making and jug choked dozens of barrels. (I no longer do any M/L work.)
