The normal discs for 355mm x 32 mm are cp 3581 family , and are floating discs, the diagram above shows these, the slot indicates the need for a "Bobbin " kit to be used to allow the disc 0.4mm movement on the bell .
If im wrong and its a "hole" in the disc , then the nut is usually fitted away from the hub face pointing inwards so it dont catch anything on the bell face .
hope all is clear !