I have a Ithaca Flues S/N 319xxx, on the underside of each barrel in front of the pivot lug next to the proof stamp they are marked 13 1M. I am not a gunsmith but I believe they are both modified. I would bet that is the stamping. I want to remember that I was told the rear trigger is modified and the front trigger is full, since the correct firing order would be rear then front, or modified then full as the game is going out of range.
I tried firing the front first only one time and ended up doubling on the recoil.