i think kutter nailed this, and daryl has supplied ample corroborating evidence - this is an H & R gun separated from its original barrels, almost certainly a 10 gauge, and was rebarreled with a liege supplied barrel set. the work was done stateside, thus no proofmarks were required - supplier had no 10's, but could step a 12 up to match the action width. seller is trying to spin it up to be the remains of a super rare two gauge set (unheard of! never before seen!), or the result of some exotic collaboration by two very famous makers.

slick eddie's used cars & fine guns....

"it's a poor sort of memory that only works backwards."
lewis carroll, Alice in Wonderland