It's a top-mount spool holder for the Robo3D.
It mounts at the top of the case, using the filament slot.
Print parts "right, left, middle.stl" and choose if you want the plain spool holder top part or the one with two holes for hanging your most used tools(tweezres/spatula..?)
Push towards either end of the slot until it sits firmly.
Have been using it for weeks and even though I have taken it apart and reassembled it numerous times, still functions reliably.