['68OrangeSunshine]

Back in the day when the truck was new and still under warranty, the Protect-O-Plate functioned like a charge card for warranty repairs. The owner would present it to the service counter, they would mount it in a special slot on a clipboard/roller-gizmo that would hold a special triplicate paper order form and slide a rolller over it all, thus leaving all the original Owner's warranty info, and pertinent numbers on the form, without having to write all that in. I guess that was so the dealer got credit from GM for performing warranty service. [IDKR]
Most 2nd owners, if buying the truck after the warranty period, would chuck them out while cleaning out the glove compartment, as they were no longer any good to them.
Now they're just rare nostalgic items. More valuable to you as a sentimental family relic.