My 16 gauige frame is marked XX. I have also heard of a few lightweight 12 gauge guns built on the XX frame, though I don't have one. My 16 on the XX frame weighs in at 6 3/4lbs while I have a 12ga not on the XX frame which only goes 6 lbs. The Bar of the frame is slightly slimmer on the 16 XX but the breech is a bit larger on the 12, though the barrel walls are thinner. Both are H grades with twist barrels. None of mine have a mis-match lik e that including this 12 which is a very late Ithaca gun which also has a mis-match on SN's from the frame to barrels (4 numbers apart).
