I've figured out most of the features of a small dividing head from eBay
except for a flat, black screw head on the side opposite the crank, near the
bottom when the chuck axis is parallel to the base. It's about 3/4" in
diameter with a straight slot that nicely accommodates a quarter (US$0.25
piece). The screw only turns about ninety degrees before it stops in both
directions, and no jiggling of the crank seems to make a difference. I was
hoping to find a way to disengage the worm for using the direct indexing
plate but don't know if this is that feature. One more detail: the screw
looks just like the gib adjustment screw on my mill. Thanks for any help.

Buddy