Might be the screw and the clip was glued to the helmet (the post ROTJ helmet already had both straps missing and who knows in what way they modified the interior), but if you looks at the Pre-Pro #3 image (in the right down corner of the ROTJ image) there's no screw visible.
View attachment 207220
Another image of the mock-up (disregard the inaccurate screw):
View attachment 207221