The large crane boom emanating from the front of the turret identifies this vehicle as an M31. Disguise was attempted, however: a dummy 37mm gun barrel attached to the turret rear and a dummy 75mm gun conspired to convince the enemy that this was a regular tank. When stowed, the crane boom would assume a more horizontal position. Stowage boxes are mounted on the rear, and tow cables snake along the hull side. (Picture from TM 9-2800 Standard Military Motor Vehicles.)
Although appearing much like a riveted medium tank M3, this is an M31B2 recovery vehicle. The dummy 75mm gun barrel is mounted on an access door, and the face of the turret would normally be occupied by the crane boom. The circular weld marks on the front hull show where pegs for stowing extra road wheels had been attached. This tank has both of the hull machine gun ports, but in US practice only the inboard machine gun would be mounted. The recovery vehicle variant lacks the characteristic M3 machine gun cupola, and this machine appears to be wearing T49 tracks.
A closer look at the door and dummy 75mm gun is provided here. The weld marks for attachment of a grouser stowage box can be seen below the driver's port; further grouser boxes were mounted on the roof above the dummy 75mm gun and on the hull roof above the driver's port.
The circular weld mark on the rear of the turret indicates where the dummy 37mm gun was attached, and the turret would normally be reversed so the crane boom rested on the hull rear deck and also so this dummy gun was trained forward. Stowage boxes were mounted on the side sponsons, and the remnants of their attachment can be seen around the outside of the rivets.
Although labeled as an M31, the engine exhaust and rear hull armor arrangement show that this vehicle was based on the M3A5, meaning that is actually an M31B2. A towing pintle has been added below the engine exhaust.
This vehicle is more complete, and allows us to see the towing pintle on the final drive and differential cover, the spare road wheel mountings, and the grouser boxes on the front hull and hull roof. A dummy 37mm gun is present as well. ("M31-ARV-batey-haosef-1.jpg" taken on 2 Oct 2005 by Bukvoed. Used under CC BY 2.5; cropped and resized from original.)
The rear stowage boxes are mounted, and the crane boom on the face of the turret is stowed to the rear of the vehicle. ("M31-ARV-batey-haosef-2.jpg" taken on 2 Oct 2005 by Bukvoed. Used under CC BY 2.5; cropped and resized from original.)
The rear armor and engine exhaust allow us to identify the base vehicle as an M3A3. Spare track blocks were stowed on the rear stowage boxes, and a better view of the crane boom and its support jacks is provided. ("M31-ARV-batey-haosef-3.jpg" taken on 2 Oct 2005 by Bukvoed. Used under CC BY 2.5; cropped and resized from original.)