Marines conducting jeep stacking experiments, Quantico, Virginia, 1941.
The jeeps are two very early Willys "slat grills".
The top jeep is USA Registration 2033271, making it a November 1941 jeep - first month of MB production. The tire from the top jeep blocks out the lower USA number.
Note the modified "deep water fording" exhaust on the lower jeep.
Photo Credit: Defense Department (Marine Corps)
Date: 1941