[Lynn Eades]

Alex, maybe usage and tyre profiles changed them? In post #6 on the left they are more shallow than those on the right. The design looks like the angled outer edges would "hold" the sand in place as the weight pushed them down. They would have been designed to carry the weight with the "assistance" of the captive sand, and no more. As some would know, running a really low tyre pressure gives a much larger foot print and therefore much better traction. With time contraints, the load, and the available equipment, sometimes decisions would have been made against deflation.
Depending on how these were used would dictate the shape after some use.
Maybe the narrower square sided ones were just for cars?
I would think that when the truck ones were sitting on a flat surface, the center and both edges would be touching the ground or that the center would be just clear. Stacking and stowing would be easier.