I've never seen an M12 that doesn't have the shells stamped on the side of the barrel, the left side just in front of the chamber. I've also never seen one that wasn't a 2 3/4" although maybe they made some 2 1/2" chambered guns. The 3" guns are clearly marked as such. Do 2 3/4" shells cycle properly? Do the serial numbers match? What numbers are they, maybe the chamber can be tracked the number.
As to what you can shoot, we would also need to know the choke... Lots of the old M12s are full, and you want to avoid shooting steel through them.
If it was my gun I'd drive in to Simmons and have them look at it... they have volumes of experience with the various configurations of M12s and could tell you exactly what you have, and what you can do with it.