Going from memory here but as I recall British proof required the Choke marking on any barrel having .008" or more constriction. As the right barrel is unmarked it would seem it did not originally have any more choke than the current .007". The left barrel would have had more choke than current. That 5" of taper to the choke in the right barrel is longer than would normally be expected.
Several possibilities exist. My best guess is the barrels have been cut, the bores enlarged, at least the right one, leaving that long taper & then the left barrel for some reason jugged instead of leaving chioke as was done on the right barrel.
Perhaps as Ted suggested it was done with the intention of having the right barrel the tighter choke & the left was made cylinder & at some later date it was deemed desirable to have a bit of choke in the left as well & the hug was added.
A measurement of current bore diameter would be a big help to see if they are compatible with the 13/1 (.719"-.728") mark.
