All postal vehicles have a license plate or two. It's required. However, they issue their own plates and their own numbers. They do not use state plates except special circumstances ~ like running about in South Philly or Wise County Virginia ~ and some places in North Carolina ~ you wouldn't want to give yourself away with a gub'mnt plate.
Look for the inspection sticker ~ it ain't there either.
Department of Defense doesn't hang plates on tanks either.