Jacobs super chuck question

I have a knurled-sleeve Jacobs No. 1 Super Chuck which came
to my hands among tooling for a lathe. The chuck has a small
slotted screw in back end, near the outer perimeter, roughly
parallel to the shank.

Anybody have an idea what it's for? I don't think any Jacobs
chucks used screws from the factory. First thought was maybe
a rotation stop to prevent putting a wildly-wrong drill in
a production setup, but the chuck adjusts full range.

The question is mostly a matter of curiosity. The chuck looks
rather beat up, though it's not seized and the jaw surfaces
aren't obviously mangled. I'm not likely to use it soon....

Thanks for reading, and any ideas.

bob prohaska