The piece is an L shaped plastic deflector (GM called it a BAFFLE) that came on all airconditioned V8 Pontiac A-bodys in '71 & 72. Bolted on the bottom of the corresupport & helped divert air up & through the radiator for better cooling. There is a profile shot of the air dam here, 4th shot from the top.
http://www.yenko.net/ubbthreads/show...40/Main/113120