That is the actual latching mechanism, the hook looking piece on the left in your picture clips over the hoop in your other pictures - hoop on the seat bottom, latch on the seat back. The flat plate that faces you in the picture is what the button on the hinge cover pushes on the release the latch. The first picture of the latches in your initial post is a set with the hoops. When you buy a set like that you don't need the black version that you also posted a picture of.