Answer: 1988 chevy Silverado headlamp bezel.

People keep asking how I knew this....I've been a Maryland state inspector for a long time, and part of each car and truck inspection up until recently was a mandatory headlamp adjustment....If I had to guess, I'd say someone from the Hoppy brand headlamp aim checker company probably bribed some politicians a long, long time ago, which made it mandatory for each station to buy their equipment to perform inspections....Anyhow, I've done a shit-ton of headlamp adjustments, so right away I knew what that notch in the plastic was for....I first checked 1988 Ram 1500 trucks and by chance, there was a picture of a mid 80's Chevy truck.