Starrett center test indicator # 65

[Holescreek]

I picked up a similar but different tool at the flea market 2 weeks ago. Similar in that it does the same thing (used for adjusting a center punched part into center of a 4 jaw chuck), different in the way it works. With this one you put the point in the center punch indent and the opposite end on the tailstock center then use an indicator on the ball to center it up. The spring action on the shaft keeps the pressure steady during adjustment.

 

[mckay3d]

I have never seen one of these . How does it work ?
http://www.ebay.com/itm/391789471476?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

Here is a PDF with some info.