This may be the pilot M53. The arrangement of the square transmission access port covers in the front of the hull differs from production vehicles, in which the three ports were in a straight line across the hull. The end of the 155mm gun is also threaded for a muzzle brake, while production vehicles were not so equipped. Brackets for extra track blocks are fitted to each side of the turret front, and attachments for a canvas dust cover surround the gun mount. There is a machine gun mount in front of the commander's cupola on the turret roof's right side, and the circular support above the gun tube is a telescope cover support that allowed the gunner's telescope M99 to see through a hole in the canvas dust cover.
The vent in the side of the vehicle above the drive sprocket is the front air-exhaust duct, while the mesh screen just in front of the turret is the rear air-exhaust duct. Stowage boxes are placed between these ducts. Attachments for a fender extension run along the side of the vehicle just above the tracks.
The air exhaust and stowage box setup was repeated on the vehicle's left side. The outline of the driver's doors can be seen in the turret just above the final track return roller, and his roof hatch is on the front left corner of the turret.
The large stabilization spade dominates the rear of the vehicle. It normally is retracted to an almost vertical position, and hidden by it in this image are the towing pintle and the rear stowage compartment access doors. The rear doors in the turret can also be seen.
The gunner's doors were built into the right side of the turret, and the commander's machine gun pintle mount can be seen just above these doors.
Minus the driver of the accompanying 5-ton truck, the crew of the entire gun section are shown here ready for action. The abbreviations are as follows: D: Driver. G: Gunner. CS: Chief of section. AS: Ammunition specialist. AH: Ammunition handler. The numbered crewmen are cannoneers. Cannoneer number 1 is also the assistant gunner. (Picture from FM 6-93 155-mm Gun M53, Self-propelled and 8-inch Howitzer M55, Self-propelled.)
The crew of the gun section are shown mounted in this image. Six men rode in the self-propelled gun, leaving 6 in the truck. (Picture from FM 6-93 155-mm Gun M53, Self-propelled and 8-inch Howitzer M55, Self-propelled.)