Re:50 STATES, 50 SANDWICHES!
Born and raised in the Commonwealth of Massachusetts, I will agree with the Fluffernutter. My other thought would be the Clam Roll. Since it is on a hot dog roll, I don't think of it as a sandwich. But...you do find it under the sandwich heading on a menu.
At our house, my Mother did experiment with the basic Fluffernutter structure. A special treat was on raisin bread toast. Sometimes my brother would add grape jelly. My sister tried strawberry jelly before the strawberry flavor Fluff came out!!!