A fun fact about magic items that cast spells is that in every case I've seen they are listed as one rarity higher than a spell scroll for that spell. Enspelled Armor/Weapon/Staff starts as an Uncommon for Cantrips and First Level. The Hat of Disguise (level 1 spell) is Uncommon, +1 rarity compared to the scroll. Boots of Levitation (level 2 spell) is Rare +1 rarity. Necklace of fireball (level 3 spell) is Rare. Wand of polymorph (level 4 spell) is Very Rare. It's always +1 rarity from spell scroll. Judging it based on every other item that the ability is "cast X spell" this helmet should be Legendary, because that's +1 rarity when compared to the same spell on a scroll.