If you have a Commodore USA produced 64x breadbin style PC you're trying to put together, this part is the fan vent for the back of the machine.

This vent is designed to be coupled with a 40mm x 40mm x 10mm fan.  This fan slides into the pre-designated location on the back of the machine, and this vent slides in to protect the small fan's blades.  I've designed it to provide maximum fan protection with a minimal amount of blockage of the fan area to allow for maximum flow.

The design is visually modeled after the original photographs that were found on the Commodore USA website when it was still online.  This I believe is the most accurate representation of this vent available.