when I replaced the original carpet in my '69Z28, under the carpet, I found a brand new key, with the numbered knockout intact. It was directly under the steering wheel.. I tried it in the ignition, it fit/worked perfectly..
On a '57 BelAir, I found a factory 'paint sock' under the carpet.. it had the same blue color dripped down the side as my car. A 'paint sock was used on the paint cup to prevent dripping onto the car. (In '57, the cars were painted manually...
It almost seems that the factory worked intended to put something unique from the factory under all the carpets..