This was something of a small continuity error; as an air force general, Denning would have been driven around in an air force Jeep, which was often executed in dark blue, though green was commonly used also. Either way, the registry should have read "USAAF" (United States Army Air Forces as the branch was still called in July 1947, until it became the United States Air Force in September of that year), instead of "US" as it did in the episode. For completeness sake, US Navy Jeeps were executed in light blue and carried "USN" registries.
During World War II, the Jeep had a German military counterpart, the Volkswagen Kübelwagen. With over 610.000 units, the Jeep has arguably become the most widely war-produced military vehicle, of any type or by any nation.