There are tabs that face front and back.
I used a plastic pry tool to get up under the middle of the long side of the cap. You could also do it with a flat head screwdriver - just be careful not to scar the plastic.
If you zoom in on the photo, you’ll see the cover with the tabs.
I installed similar D rings, but used all stainless steel rings and hardware. The D-ring holes line up perfectly with the holes in the bed rail. Bolts are 1/4” wide and 3/4” long. Attached underneath with a fat washer, lock washer, and nut. I used rubber gasket material between the D-ring and...