This is a single hot end mounting bracket. The hot end heat sync will bolt to the bottom of this bracket using three screws that can slide in and grip the heat sync grooves. This bracket is made to be used with the spring loaded effector design, similar to the 3 head version. The small holes are for wire ties that keep the hot end wires tied nice and tight to the bracket. The other photo shows how a J head is mounted on this bracket and how the bracket mounts to the effector using the spring loaded screws. This has been tested with E3D hotends too.