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Ralf C Kohlrausch,
Thank you for that inrtiguing bit of history on the Davis sextants. I enjoyed the idea of the physical address burned into the casting. This particular instrument seems to date from the transition period. How many years later? I don't have a guess...

Regarding, the "mark" numbers, you wrote:
"The Mark X was Davis' first "Master Sextant" with the looks of the real thing and the same vernier as the Mark III and only a sight tube." 

Thanks. That makes sense. You may notice that the photo I included previously shows that sextant with a small scope, not just a sight tube. However, the standard manual included in the case, which is probably (not certainly) original to this sextant, has cover artwork that seems to indicate an instrument with "only a sight tube", consistent with your deescription. In this artwork and in other photos I have seen, there seems to be no way to remove the sight tube and swap it for a scope. But the manual has a half-page insert tucked into it! This seems to have been printed after the original manual was produced in quantity. And the insert explains that the sight tube can be removed and replaced with the little scope. At the top of the insert, it says "m.12" sextant.

So here's a theory: the Mk 10 sextant was the original "master sextant", and the Mk 12 is nearly identical but includes the small scope. In addition, the mounting point for the sight tube or scope on the later sextant has the small --and rather sharp-- "jaws" that we also find on later Davis plastic sextants. Apparently the Mk 10 did not have these jaws, and instead it appears that the sight tube was attached more or less permanently (maybe glued in place?).

This sextant (the one I am referring to as a "Mk 12" here) is in good condition now, but you wouldn't guess as much on first look. It was one of those where the foam padding insert in the case had chemically decayed (maybe there's a better word for this?). It crumbled on contact. When I lifted half of the insert out of the case, it fell apart under its own weight in my hands. Very messy and the material was somewhat sticky covering the sextant like a dark green fungus. Fortunately I had experience cleaning up one of these, and I knew that debris would come off with a little spray of water in the kitchen sink. I also had already set up tooling to make my own replacement foam inserts, so within an hour I had the poor old thing looking great! There's only a little rust here and there around the adjustment thumb screws to hint that this sextant is more than fifty years old...

The simple knob for adjusting the index arm on this Davis Mk 10/12 sextant is inferior to the traditional adjustment screw on 19th century octants (which are otherwise similar). The knob on this Davis sextant has to be nudged with the gentlest of touches to get it to move by some useful small angle. And compare against a Mk 15 with a micrometer and worm gear. On those later sextants, one full turn of the micrometer drum equals one degree change in the observed angle. The knob on the Mk 10/12 shifts the angle by about fifteen degrees in one full turn. In practice sights, since "live" adjustment was so sensitive, I found it easier to lead the Sun a little bit and wait for it to arrive at the angle as set. This worked well.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA