halter and bit
勒 centers on a bridle's metal mouthpiece and the leather straps that control a horse: a halter and bit together. The idea is restraint and guidance through a head harness.
勒 combines 革 (leather) with 力 (strength/force), suggesting a leather device that exerts control. The exact historical development is uncertain, but the component combination fits the meaning of a bridle or halter.
Leather 革 straps plus force 力: imagine a strong leather halter with a metal bit that lets you control a horse.
For ロク, picture a horse locked into its halter and bit: lock -> ロク, and the leather straps keep it restrained.