An Apollo Lunar Sample Return Container (ALSRC), or "rocks box". This is
ALSRC-12 (serial number 1012), which flew
on Apollo 15 (page 7 of that report [p. 13 in the PDF] contains a
post-flight photo of the ALSRC).
Lunar surface samples, most of them in sample
bags, were placed in a sample return container such as this. The ALSRC
sealed and maintained a vacuum to prevent contamination of the samples.
To prevent contamination of the command module, the ALSRCs were cleaned in the
lunar module and placed in a Lunar Sample
Container Decontamination Bag prior transfer to the command module.
I photographed another ALSRC at the National Air & Space
Museum, and also at the Destination
Moon travelling exhibit.