This cover belongs to an Oxygen Purge System used during the Apollo 17 mission that occurred in December 1972.
The PLSS was used for water and air circulation, communications, and oxygen "scrubbing". It weighed approximately 65 lbs. and all the units used during the Apollo missions were left on the lunar surface due to weight considerations.
The OPS was attached to the upper portion of the PLSS. It was used for oxygen storage and circulation. Apollo 17 Commander, Gene Cernan, brought the OPS and its cover back from the surface of the Moon to conduct what was known as a "pop-up" EVA to be conducted from an open hatch of the returning Apollo 17 command module.
NASA transferred this item to the museum at the conclusion of the Apollo program.
This object is on display in Destination Moon at the National Air and Space Museum in Washington, DC.