Screw cap

These are small "wingnuts" with a through hole just large enoguh to fit around a #4 screw.
Use case:
You mount a shelf in a cabinet, but the backing is made out of thin wood veneer rather than plywood. So the screw sticks out the back, and you don't want the wood to be stripped. So screw this on in the back of the cabinet for extra grip and support.