Startech racks come with cable management rings that attach to the sides of the rack. The mounting holes for them are only on the sides ; I needed them on the front and back instead. This right angle bracket attaches to the mounting holes on the side and offers the necessary mounting hole for the ring on the front.
These brackets are mounted on the rack using M3 screws and nuts. The rings are attached on the bracket with the same screw they come with.