This listing is for a fresh a/c cowl vacuum assembly (GM Part #3979732) for all 1970-81 Trans-Am/Firebird & Camaro/Z28.  This is the part that mounts in the top of the cowl, underneath the passenger windshield wiper arm. 

It's function is to block the air flow from the top of the cowl and into the kick panel area for the a/c recirculation mode. 

The vacuum actuator works perfect in multiple modes and the rubber around the door, which often dry-rots & disintegrates to allow unwanted air inside the car, is in perfect condition except for some pitting on the top bracket that's mostly hidden when installed!