Correct.
Like nearly all British game guns, the left barrel would have been choked to some degree. The left barrel is marked appropriately for such choking. As noted, it could have been anything from what we call skeet through full.
The 12C marking is the standard English mark for a 12 bore shotgun with a nominal bore. It has nothing to do with choke.
That they are .729 means the gun is indeed in proof. Excellent. The only question would be is whether the choking was polished out or the barrels were cut. If they still touch at the muzzle, it was polished. With cyl chokes, it is a fine upland gun.
Add to that
@Red Leg , a “plunge gauge” you drop in the muzzle to measure choke isn’t worth the $40 it cost to make. A $200 bore diameter gauge may find that there is more choke present 1/4” into the muzzle. I’ve bought many English guns that were allegedly Cyl/Cyl and when properly measured in the barrel with a bore diameter gauge rather than a plunge gauge, I found anything from Cyl/Skeet all the way to Cylinder and 3/4 choke. (Improved Modified)
Nothing of this points badly to the gun, just an academic discussion on what a gun really might be beyond what the proof marks say it is. To get to such facts after learning the very hard and expensive way, I went out and bought $2000 in gauges to measure chamber, bore, choke, and wall thickness and then all the archaic books that define the various proof marks to a level of detail not found on the Internet. It matters very little on a $3000 box lock but these nuances can create $10,000 price swings on best guns.
