The truck sheets were apparently available in small quantities beyond the one-at-a-time that were handed out to visitors to the truck.
Shown below are examples of stamp clubs overprinting the truck sheets for their local shows.
These are the only four varieties that I am aware of.
They almost certainly used an inexpensive hobbyist letterpress, such as the Kelsey 3 by 5 inch model shown below, to overprint the text. These sold for around $100, with type and accessories, back in the 1940s.
All of these overprinted sheets are considered rare in the philatelic exhibition label collecting community, with each one selling in the hundreds of dollars.
The examples that you show were probably similarly produced, but as the sheets had no value as a postage stamp, there was really no reason to precancel them.
All of these items are rarely seen.