Enlarge the bracket of the wooden box and insert the key you obtained at the bottom of the elevator into the key hole to open the box.

Turn the dial to check that there is a ✕ mark.

Check the picture drawn directly to the right side of the wall where a wooden box is placed.

The position of the ✕ mark is aligned with center section of the dial. Adjust the dial to the ▼ position.

Adjust the top section of the dial to the ▼ position based on the picture drawn on the left side of the desk's tabletop.

Adjust the lower section of the dial to the ▼ position based on the picture inside the frame hung on top of the workbench where there is a knife stuck into it.

Press the right switch last.

Get the spanner from inside.