These are the factory caps located in the hatch of your 87 GLHS.  When you open your hatch, you should see a pair of grey interior panels (hatch floor area).  Those large dark grey decorative interior trim panels hide the trunk area and your rear adjustable Koni adjustable shocks.  (assuming your GLHS still has them). 
 
There should be a small black cap that covers the access hole to the top of your rear koni's.  (so you can adjust them with the Koni knob you should have in your glove box, or center console).  If your GLHS is missing them, here's your solution.  These are the original caps from Shelby Automobiles.  They've been in this bag since 1987.  Price is for 1 cap.