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Yes, that's my understanding of this component. I think I asked the same question a couple of years ago...

There is some simple logic at work. Have you ever had a micrometer drum on a marine sextant come loose? On many sextants, including high-quality instruments, the micrometer drum is held to the shaft of the worm gear by a screw. If that screw is loose, the micrometer drum will turn, but the worm gear does not turn. Round and round... but nothing happens! Even worse, if the drum is a little loose, the drum turns in fits and starts. Quite frustrating. But of course, we can take advantage of this property: if there is an index error that is difficult to adjust, we can reset the micrometer itself to cancel out index error. Just loosen the micrometer drum and turn it to whatever position yields zero index error. On these C&P sextants (it's a C&P, right?? Not sure... out of my price-range), this extra knob enables this micrometer drum adjustment as a feature. This resurrects the traditional "easy" cancellation of index error before a round of sights that was normal on 18th/19th century octants (big knob on the back behind the horizon mirror) and is also found on most plastic sextants. Rather than assessing index error and adding an arithmetic step to each and every sight reduction calculation, we just zero out index error before the sights. This is what I recommend whenever possible.

By the way, isn't that the sextant of that Finnish Golden Globe Race entrant? It circumnavigated and recently was sent to a sextant adjuster to be refurbished and cleaned, ready for the next GGR??

Frank Reed