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This image may help.  The source is an old auction via Invaluable.com.

We can see that the index arm is not attached at the left-hand end (as viewed in the photo).  The cover over it is (my guess) merely for protection.

We can also get a sense of how the spring device above the index bar, on what resembles the index arm of a sextant,  exerts downward pressure on the index bar against the upward pressure of the screw mechanism.

I wonder whether the example in this photo may need adjustment.  In other photos, with the index bar set to about the same height, the end of the bar appears to be covered or mostly covered by the protective cover.

Also of interest may be the original patent for the Fiske stadimeter (attached) - which although it has a different appearance operates much the same way but without the advantage of the rapid adjustment.

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