According to the write-up on the Mystic website -- "These cards, depicting the White House, were actually printed aboard the truck!"
That is not correct. All the Philatelic Truck sheets were printed by the Bureau of Engraving & Printing in DC. However, some were packaged in rolls, like coil stamps, and run through a miniature demonstration "press". This chopped off the souvenir sheets but did not actually print them. Unfortunately it was slow and sometimes jammed, so in order to move people through the truck more quickly individual sheets were just handed out at the end of the tour.