This is a wider washer for Dracula's Tower Puzzle Box that covers the screw coming up from the slider and provides a flatter base for the puzzle core. It helps to avoid snags on the goodies and things you'll surely put inside the tower!
(Note, the photo is from an older revision of the model, the new one will let the screw sit inside a bit to take up less space.)
The author remixed this model.
This wider washer will cover the screw coming up from the slider to avoid snags on the goodies you'll put inside the tower!