Coolant Reservoir for 76 Plymouth Duster

Tooljunkie said
Hose goes to resevoir. Essentially it is intended to keep rad full.
Coolant is drawn from the bottom so it doesent pull air into rad.
Be sure to check cap, you need the overflow type. Anything can be used that will hold a satisfactory amount of coolant.

Thanks! What is the round and black part that is on the very top left of this photo? It has a hose connected to it. On my car it is closer to the radiator and I thought it may have hooked up to the reservoir.