GBody Buick, Olds, Pontiac Radiator overflow reservoir. (NEW GM)
Note: All Chevy over flows are the same. All B.O.P. are the same. GM only makes 2 units. They may not be 100% identical to what you had in the 80's. The Buick unit shown is what all B.O.P. use now from GM.
Chevy owners you will have the same one you had back in the day.