In the PCIe specification (see page 75), the "short arm" of the bracket has two cutouts; one for the slot that the retaining screw fits in, and another square cutout that has additionally had the sides bent back at a diagonal angle. Actual brackets also have these features. However, at least some slot covers omit the diagonally-angled bends.
What is the purpose of this cutout, and especially, the bent bits?
For bonus points, what is the purpose of the round hole at the opposite side of the bracket that some brackets have, and some don't?