This is a socket for the bolts/screws of the “customized sanding sticks with labels.” Using this, you can tighten the screws without tearing up your fingertips or printing entirely different screws. 

I've included the blender file if you want to edit it at all. 

EDIT: Seems a few people have had trouble getting the thin walls to print. Technically they're not necessary as it'll work fine without them, but make sure to enable “print thin walls” or similar settings in cura or your slicer of choice before you print. Again, it'll work without them, but I think it'll be slightly stronger with them!