This is a model for a low profile shelf bracket.
It is designed to slide a ~7mm shelf into it and secure it with a screw from the bottom.
The hole for the wall mount screw has a 4.2mm (7.8mm screw head) diameter and the hole for the screw to secure the shelf plate has a 3.5mm (6.4mm screw head) diameter.
It cannot hold a lot of weight but blends in nicely without being too visible.