That really is an elegant one Chuck. As previously mentioned, most probably just a tensioning nut.
Just to explain what they do and how they work.
Imagine you have a pop mark on a workpiece that requires setting up in the 4 jaw independent.
You put the part into the chuck, and stick the short pointy bit into the pop mark. Then turn the lathe on at a fairly low speed, and then move the cross slide until you get the lowest amount of wobble on the long sticky out bit.
Then it is a matter of adjusting the chuck jaws and cross slide until the long bit stops wobbling. You then have the pop mark on lathe centre.
The normal way of making one is say a 1/2" thick plate with a rod silicone sealed into a larger sized hole. Once the silicone is set, you have a wobbler.
Pic should explain a few things.
Bogs
