The acronym RSO stands for Raupenschlepper Ost, which translates as ‘Caterpillar Tractor East’. The terrible muddy and icy conditions experienced on the Eastern Front convinced Germany it needed a highly mobile, tracked vehicle. Steyr proposed the RSO and it immediately became popular thanks to its mobility and reliability. In all, about 23,000 RSOs were produced by a range of manufacturers. The original version had a pressed-steel cab and truck-style cargo bed. Later it was decided to convert some into an anti-tank role by mounting the 75mm PaK 40/4 gun on the rear and by lightly armouring the driver’s compartment.