So you're saying it's a dealer installed option, or accessory. It makes sense.The '76-'81 Chevy parts catalog list glass mounted add on "Lamp Package, High Level Stop" for '80-'81 as a fits-all (Group 21.298, p/n 999405). I do remember seeing them stuck onto back windows in the early/mid '80's. These was a fits-all for everything back then. Like every thing else from back then it's long discontinued.