Coolant recovery bottle
Its a California car (purchased new) for some reason Hamtramck put a fender mounted (RH) right near the alternator. It was held by one bolt ring mounted around the neck of the bottle. Never have seen one ever. Im looking for the the one mounted next to the right side of the radiator.
Here's a pic of the bottle I have (OEM) but this one pictured has it mount using a block of wood and mounting on the rad support.