That could be it -- but IIRC, it is near the other end of the worm screw from the hand crank and index plate. It both allows the screw to be disengaged (it is in a bearing which is pivoted near the index plate), and to be adjusted for minimum backlash when it is engaged.
Hmm ... I've made some by using a hex collet and hex stock, turning the OD down using a box tool (in a bed turret) and threading using a Geometric die head, then parting off. Very quick if you need to make more than one -- slow to set up for the first one.
Yep. My dividing head has a center with a T-bar just behind the point. The T-bar is slotted parallel to the axis on each end, and drilled and tapped for a square headed setscrew on each end. You set up the work between centers, clamp a dog (also using the same style of square headed setscrews) around the arbor or some direct part of the workpiece, and then tighten one setscrew in the T-bar against the tail of the dog so there is no give in the connection of dog tail to T-bar.
It also has a hefty three-jaw chuck which screws onto a 1-1/2x8 (or something similar -- I really need to check that again and make a mount for the smaller 4-jaw chuck which I used to use on the old 2-1/4x8 spindle for the Clausing before I converted it to L-00 spindle nose.)
And, of course, something to prevent rotation of the workpiece relative to the dividing head.
Right -- but I don't have a faceplate for the dividing head. Perhaps I should make one of those, too. The thread should not be difficult to turn using the Clausing.
Enjoy, DoN.