NavList:

I don't know if you saw Bill's link just following the one you mentioned:

http://sextantbook.com/category/evolution-of-sextant-micrometer/

From there and messages on the list from various years I've seen numerous mentions of drums without micrometer, including the Heath "Endless tangent screw", and a picture of a presumably non-endless screw from 1847. All these had arc verniers, of course.

In 2005, George Huxtable found reference to a sextant by Jesse Ramsden, around 1787. That, as I understand, had a micrometer-like shoe that could clamp at arbitrary positions and add or remove fractional amounts from readings on the arc vernier.

In that same message, George quotes Herbert Prinz to refers to the 1908 Trommel-sextant from C. Plath.

Later in NavList 1412, George wrote some more about the 1908 one, and another Plath from 1922 that had micrometer and vernier, but I couldn't tell if that meant drum vernier or arc vernier. Also mentioned was a Heath from "late 20's".
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