The M-923 Truck, Cargo, 5-ton, 6x6 is part of the M-939 Series of 5 ton 6x6 trucks. Details of the history, model numbers, and configurations of the M939-series of trucks are on the linked page.
The M923 is a dropside cargo truck of standard wheelbase. The M925 is the same vehicle equipped with a winch. USMC M923 (and M925) cargo trucks had an updated dropside cargo body with five drop sections (one rear, two on each side), compatible with ISO containers.