The Raupenschlepper Ost, literally “Caterpillar Tractor East”, is more commonly abbreviated to RSO. This fully tracked, lightweight vehicle was conceived in response to the poor performance of wheeled and half-tracked vehicles in the mud and snow during the Wehrmacht’s first winter on the Soviet Front. Steyr responded by proposing a small fully-tracked vehicle based upon its 1.5 tonne truck (Steyr 1500A light truck) already in use in the army. The vehicle was introduced 
in 1942 as the Raupenschlepper Ost (RSO). It was initially designed as a prime mover and artillery supply vehicle but eventually served in a wide variety of roles. By 1943, the complaints of the infantry anti-tank units at the front that it was almost impossible to move their guns using trucks at daylight under enemy fire, leading to enormous losses of equipment when an emergency relocation (at the time more of a euphemism for withdrawal) was necessary found their way to the top. This led OKW to consider an older proposition to fit the 7.5 cm PaK 40/1 anti-tank gun -by then the standard Pak- on top of a RSO chassis and Hitler after seeing the blueprints ordered a limited production run for combat testing even before the test vehicles were completed.
