Secret Compartment '72 Demon

As others have said that is your passenger side fresh air intake. It takes air from the grill on the cowl. As to years I know 67,68,69 have them, probably up to 76 if I had to guess. My 56 ford pick up had one that opened at the cowl. I suspect that even 30s or 40s cars had them. They went away when integrated AC systems came along and incorporated fress air intakes to the system.

Joeycurl, just curious, where you fit age range wise 20 to 30, 30 to 40, 40 to 50?