81mm Mortar Carrier M21.
The M21 was very similar to the 81mm mortar carrier M4, but the machine gun and mortar have swapped positions in the passenger compartment, with the mortar now facing forward and near the front and the machine gun on a pedestal at the rear. The side stowage racks are not mounted on this vehicle. (Picture from TM 9-710 Basic Half-Track Vehicles (White, Autocar, and Diamond T).)
This carrier does have its side stowage racks mounted above the mine racks. (Picture from TM 9-710/TO 19-75A-77.)
Stowage racks on the rear are visible from this angle, as well as the machine gun pedestal to the rear and an antenna mount forward of the machine gun mount. (Picture from TM 9-710/TO 19-75A-77.)
The frontward-firing mortar in this vehicle is located near the front of the passenger compartment, and a pedestal mount for a .50cal machine gun is at the extreme rear. The fuel tanks are located in the rear corners of the fighting compartment, and 81mm mortar bomb stowage is obvious throughout the passenger compartment. Bows for erecting the passenger compartment cover have been set up, and the mortar baseplate is stowed to the right of the rear door. (Picture from TM 9-710 Basic Half-Track Vehicles (White, Autocar, and Diamond T).)
Exploded and assembled views of the .50 caliber pedestal mount 7115438 are shown here. The mount was bolted to the carrier bed and also braced with two cross straps. (Picture from Weapon Mounts for Secondary Armament.)
