The M7 "Priest" was originally based on the M3 Lee/Grant Chassis and mounted a 105mm M1A2 Howitzer. The M7B1 Self Propelled 9.75" Chemical Mortar Prototype fitted a 9.75" (284mm) Petroleum Warfare Department (PWD) Mortar that fired incendiary shells of white phosphorus. The weapon was not accepted for widespread use being considered unsuitable by the US Army. Includes photoetched parts.