They are access covers available from time to time on Ebay and Stovebolt. They are 55.2-59. There is no wiring in that cavity. They are access at manufacturing. They are also access for tailgate cable retractors install.
Interestingly, on that wall behind the wooden rails, you will sometimes see the "shadow" of those plates. When the side of the Panel or Suburban was laying flat, the factory painter laid those covers down on the inside wall and painted them.
They have a rolled edge, hard to make. There is a gasket also.
