Here's part of an explanation of their origins; the first is from Sixty Years of Chevrolet an American book published in the '70s. The other two are from Norm Darwin's books on the History of Holden since 1917 and The History of Ford in Australia. My father was in the army during the war and was involved in the troop movements up to Darwin from Adelaide. The Americans were building the road from Alice Springs to Darwin at the time. The troops were all taken by Chev or Ford Blitzes and he had some great stories, which I wish I had listened with more attention. Apparently the Fords over heated a lot and both were unbearably hot in the cabin. In the mornings the drivers would bolt to get the Chevs first, but he said the best thing about the Fords was that if you rigged up a hot plate over the manifolds, you could fry eggs while on the move. One of my biggest regrets was not asking him more about what he did in the war (he put his age down so he could join up) as he saw all the bombing of Darwin.