Used distributor cap coil cover.  This is the plastic cap that covers the ignition coil, and screws into the distributor cap.  For HEI distributors (latch screw style).  Fits the following:

75-81 Firebird/Trans Am w/ V8 Pontiac 301, 301 Turbo, 400, 455 engine
77-79 Firebird/Trans Am w/ V8 Oldsmobile 403 engine
77-86 Camaro/Firebird/Trans Am w/ V8 Chevrolet 305 engine

The item in the photo is the actual item being sold.