head-plate
/ˈhɛdpleɪt/noun
1 Gunnery. A metal plate covering the cheeks of a gun carriage. Now historical and rare.
2 Saddlery. A metal plate on the cantle of a saddle-tree.
3 Zoology. A bony or chitinous plate forming (part of) the outer surface of an animal's head; (in an arthropod) the head capsule, or one of the sclerites forming it; (in a reptile) any of the scales on the head.
Origin
Mid 17th century. From head + plate.