Raupenschlepper Ost (RSO), 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. The RSO may have been inspired by very similar full-tracked small tractors in use in other armies, mostly originated from the pre-war light to medium series of Vickers artillery tractors.
The 7.5 cm PaK 40 (7.5 cm Panzerabwehrkanone 40) was a German 7.5 centimetre anti-tank gun developed in 1939-1941 by Rheinmetall and used during the Second World War. PaK 40 formed the backbone of german anti-tank guns for the latter part of World War II.