locking plate
/ˈlɒkɪŋ ˌpleɪt/noun
1 Clockmaking and Watchmaking. = count-wheel.
2A plate to prevent the movement of something to which it is fixed; specifically one to stop a nut or screw from turning or working loose.
3A plate on the outside of a lock through which the bolt passes when the key is turned.
Origin
Mid 18th century (in an earlier sense). From locking + plate.