These discs are used together with the threaded mandrel to make glass ring tops.
The disc is screwed onto the mandrel. The disc is dipped in the dip and fire. When this is dried you can screw on the bolt.
Now fill the hole of the bolt with some dip and fire so that no glass can enter it so it can no longer be screwed onto the ring.
Now make the ring top with glass as you are used to.
18 mm diameter