The Ithaca Model 37 is a pump-action shotgun created in 1933 and produced in 1937 by Ithaca Gun Company. It was made in large numbers for the civilian, military, and police markets. The loading involves inserting shotgun shells of 12 gauge through the loading/ejection port and pushing them forward into the magazine until retained by the shell stop. The slide release is pressed and the slide retracted completely then pushed forward. Pulling the trigger fires the gun and releases the slide for reloading.
It's a great weapon to use against raiders, beastlords and their pets, but will become less powerful when used against deathclaws and super mutants. Against robots, it can be quite good when it's loaded with 12 gauge EMP shells. Flechette rounds do make this a good back up weapon if you expect to be facing deathclaws and don't have enough heavier weapons.