large ring on a front door used as a knocker
扄 centers on a large metal ring attached to a front door, used as a knocker. Its meaning is specific to that door fixture.
扄 combines 户 (door) with 向, which likely contributes a phonetic element. The character is rare and its exact historical development is uncertain.
The outer 户 is a door, and inside 向 looks like a ring facing outward. Picture a large metal ring hanging on a front door, ready to be used as a knocker.
For ショウ, imagine a showy door knocker that makes a loud sound: show -> ショウ, and the ring on the door announces visitors.