In the picture of the bbl with the damage, if I assume that the muzzle end of the bbls is the left,,my first thought was that something was stuck in the right bore.
To remove it or at least try to get it moving by breaking it into pieces,, a tin edged rounded tip tool may have been tapped down in there.
The long leads to the bulges with rounded ends appear to me to be the imprint of such a tool being pounded into the obstruction,,but close to/or against the bbl wall.
Just my take on it from a quick look.
I do have a 12ga English SxS with riveled bbls. The damage is more of an even circular pattern I'd best describe as looking like a piece of corregated tubing though not so severely deep.
If you really like the shotgungun, price it in your mind, as a single set of bbl offering and with what ever wood and metal problems it holds.
Offer low and if it's refused, walk away and find a better example you won't have to make excuses for not spend next years vacation fund fixing.
