OneStopRestoration's Special Orange 69 Chevelle 300 Deluxe Coupe

[Mr70]

I actually used that same carpet tack idea to do the same on my 72 SS Chevelle back in high school.The carpet was pillowing badly after my cousin used it & left all my windows open overnight during a heavy rain storm.We tried hair dryers etc,but nothing worked.
Drilled holes through the carpet into the floor,and Voila!,it worked.

[Woj]

I have a 69 300 deluxe hard top that had a few of those nails in the floor. I'm pretty sure it was to hold down the rubber mat or carpet in your case. Yours is on only the second time I've seen this.