Joost, After looking at the picture I could not understand why this truck had chains to drive the wheels. American trucks of that era did not have such an antiquated device. But, now I think I know the reason. The hub in the center is the axle on the civilian version of this lorry. There would be two tyres on each end of that axle. For some reason the Army wanted to make the distance between the wheels as narrow as possible so the two dual tyres were converted to 4 single tyres driven by chains. It may have had to do with the width of the pontoon bridges that these trucks had to cross.