These rings are used by lampworkers and enamelists to build a top for a ring. For lampworkers you need a mandrel and a disc that builds the base. The screw stays inside the lampwork topper and screws onto the ring.
For enameling, you put the enameled part onto the screw (with rubber gaskets on each side to protect the enamel) and then put the hex nut on. Cool steampunk or cover the hex nut with a metal paint, etc.
6mm screw height is ideal for layering components, or use with big-hole beads & a screw top. 6mm wide, & have a gently curved polished interior for comfort. 2.5 metric threading, compatible with other European changeable systems.