3" Gun Motor Carriage M9.
The M9's basis on the medium tank M3 is obvious when one looks at the lower hull and running gear. The superstructure has been redesigned to accommodate the 3" gun M1918 in a limited-traverse mount, and the driver was placed immediately to the ordnance's left. Note the ladder welded to the side of the hull. (Picture from Tank Data, volume 3.)
