The M37 is in Rick's room of weapons. Owen Mackenzie and Juan Manuel break open the lockers and find the weapons, including the M37. When rescuing Carolina, owen uses it against the hoards of "the undead". After crossing the America´s bridge, he gives it to Juan Manuel, instructing him how to fire and reload. Juan Manuel uses it as his main weapon until he finds the Benelli M4 Super 90 in a police station weapon locker.
The Ithaca M37 is a pump-action shotgun developed in 1937, and despite losing out in military contracts to the Winchester M1897 and M1912 during World War II, would see widespread use during the Vietnam War. It is nicknamed "Featherlight" for being lighter than other shotguns. It is also very reliable, and by having both its loading and ejection port under the gun, is ambidextrous.