Minecraft wiki entry for Helmet, associated text: Unlike the other helmets, the netherite helmet covers most of the player's face, due to a nose guard and cheek plates.
The turtle shell is the only helmet that is not part of a complete set.
In Bedrock Edition, there exists a bug[5] that makes it possible to obtain a Leather Cap enchanted with Efficiency I from Woodland Mansion chests. The enchantment has no effect on the cap, however.