Armored Car T18E2 Boarhound.
This machine is the second pilot T18E2 and is armed with the 57mm gun Mark III instead of the 57mm gun M1 of the production vehicles. The M1 featured a longer, thinner barrel (52.1 calibers from the muzzle to the rear breech face versus 45.0 calibers for the Mk.III) and was also lighter, at 727.5lb (330.0kg) versus 761lb (345kg). The drivers were provided with doors to their front and side, and entered via doors above their seats in the hull roof. The British No.19 radio set would be installed in the turret bustle, and one of the jettisonable fuel tanks can be seen mounted on the hull rear. (Picture from Development of Armored Vehicles, volume II: Armored Cars, Scout Cars, and Personnel Carriers.)
Another view of a pilot vehicle shows the doors over the drivers' positions. On production cars, an aperture for a 2" smoke mortar would be visible in the front right corner of the turret roof. (Picture from Modern Ordnance Materiel.)
Components of the combination gun mount T63 are labeled here. Note that the shorter 57mm gun Mk.III is illustrated; and although unlabeled, gyrostabilizer components are visible on the lower right side of the cradle, including the gyro control unit and flexible shaft. (Picture from Weapon Mounts for Secondary Armament.)
The gun mount is shown from the outside with the gun shield fitted. (Picture from Weapon Mounts for Secondary Armament.)
This car has the 57mm gun M1 mounted; its longer and thinner barrel profile can be contrasted to the gun tubes above. (Picture from Weapon Mounts for Secondary Armament.)
