The object here is to anchor the rails so securely to the slate that they function as a unified system. The picture on the right views from below one of the three rail bolts attaching each rail to the slate. Notice the oversized circular cut out in the slate framing. This permits the rail bolt to be tightened directly against the slate. The black domed washer has serrated edges that bite into the slate for a very firm hold.
Most manufacturers use a plain flat washer.
When the washer is tightened against the soft particleboard slate framing it leaves an indentation. In time, the bolt can work loose resulting in vibration or even a wobbly misaligned rail. For more information about slate, click the slate button in the frame on the left.