Minecraft wiki entry for Argument_types, associated text: Slot Valid slot_number Mapped index
armor.chest 102
armor.feet 100
armor.head 103
armor.legs 101
weapon 98
weapon.mainhand 98
weapon.offhand 99
container.slot_number 0-53 0-53
enderchest.slot_number 0-26 200-226
hotbar.slot_number 0-8 0-8
inventory.slot_number 0-26 9-35
horse.saddle
400
horse.chest 499
horse.armor 401
horse.slot_number 0-14 500-514
villager.slot_number 0-7 300-307