Twist In Shelf Pin
This is a twist in shelf pin. This cabinetry peg is inserted into a recessed rail in the cupboard and is then twisted by 90 degrees to lock in place.
It is a replacement part that is compatible with a cabinetry system that was available around 1980-1990, possibly by Enterprise Cabinet Systems.
This is a reproduction part, please verify dimensions in the images below.
This is a single part, remember that typically you will need 4 to hold up a shelf.