Rivets are normally there on fold down and non fold down to hold the interior panel to the trap door opening in the trunk. Would be another one in the lower corner on both side. A few (smaller) across the top to hold the upper panel in place
Here are a few pictures that should help but basically the non-moving trap door cover is not attached along the outer two edges in that middle section
Examples include 67 & 68 examples. Believe both are the same concerning this subject. Always surprised they didn't use at least one bumper on each side to reduce rattles but apparently they didn't have an issue
Hole this helps