Thumbscrew on ultra-wideCS lens - what does it do?

This may be a really dumb question, but what is the thumbscrew for on the ultra-wide CS lens?

LN051 / CS17320M12

Unlike other lenses there are no markings (near/far for focus or open/close for iris), and nothing seems to move when it’s loosened. Is it just to assist with rotating the whole lens in the mount?

(No issue with the lens, it fits and works great, just want to know if I’m missing something…)

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