The M4 was based on the chassis and drivetrain of the M3 Light Tank which introduced the trailing idler. One variant was designed to tow the 3-inch or 90 mm anti-aircraft gun, and another was for the 155 mm gun or 8-inch howitzer. The rear compartment carried the gun crew and other equipment and some later variants included a crane to assist with heavier projectiles. Two types of ammunition boxes were used on all models: a 90 mm box with side “tailgates” to access 90 mm shells pigeon-holed in the sides, and a combination box for 155 mm or 8-inch ammunition.
After the war, many types of these tractors were stripped of their military components and used for log skidders and power line construction.
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