The M12 was built on the chassis of the M3 Lee tank. It had an armored driver's compartment shared with the commander, but the gun crew were located in an open topped area at the back of the vehicle. It mounted a 155 mm gun M1917, M1917A1 or M1918, depending upon availability, a weapon derived from the nearly identical French 155 mm GPF gun of World War I vintage. Limited storage space meant that only 10 projectiles and propellant charges could be carried on the vehicle. Only 100 vehicles were built: 60 in 1942 and a further 40 in 1943.(courtesy of Wikipedia)