This is a press molded heater box cover, made of black fiberglass like original. It has the backing plate installed and is ready for your 1964-1967 Corvette. If you have a broken engine mount and accelerate at a fast rate, the engine can move enough to break this cover. Over tightening is another reason for this cover to be broken. Also, when removing the engine, be careful not to hit heater box.
Learn more from the following Mid America Motorworks Knowledge Library article:
Corvette Body Components from Fiberglass to Resin Transfer Molding