Just been out into the shop and pulled one to pieces.
Groove and pin easily visiblethe second diameter which can be anything is as long as the clamp is thick. Couple of smaller ones of the same build up as completed items.
The smaller ones are not three diameters but actually a length of studding loctited into a knurled boss for the head, the groove is machined into the thread.
Simplifies the making as these are to be used as opposed to learning different operations as you had to as an apprentice.
If you had access to a length of knurled bar these could be made with hand tools and a drill, no lathe required.
John S.
EDIT for some reason it's not locking the aspect ratio of the picture, this has to be the crappiest web software I have come across
Edited By John Stevenson on 04/03/2012 23:08:43