Full-track Prime Mover M33.
The M33 was a tank recovery vehicle M31 that had its turret, winch, armament, and crane removed. This was used instead as a tractor for the towed 8" gun and 240mm howitzer until the 38-ton high-speed tractor M6 could be fielded. Canvas covers were carried for the turret and machine gun mount openings. (Picture from Catalogue of Standard Ordnance Items, second edition 1944, volume I: Tank and Automotive.)
