Hi, George,
It is a Rotur mandrel.
It is then pen blank that moves around the mandrel shaft.
In order to get the knulled nut on the thread I have used two sets of bushings, parallel sides and tapered. I have noticed that the mandrel shaft is held on by an allen grub screw, so I am going to see if I can shorten the length and use only one set of bushings.
Stephen