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You wrote:
"Speaking as a photographer, there's a lot to be said for a variable
polarizing filter.  OTOH, most good sextants have filters that are
effective at dimming glare along the horizon, below the sun.  I
wouldn't go out of my way for just this feature."

Right. No reason to go out of one's way for this, but they do work. 
Incidentally, some sextants included these polarizing shades as far back as 
the Second World War. You mentioned horizon glare... While some polarizing 
filters are used to eliminate the reflected light off the water (which tends 
to be polarized in a way that can be easily filtered), that's not what's 
going on here. These filters are crossed polarizers. The idea is that you can 
vary the angle at which they cross, by turning one of the pair, and reduce 
the total brightness of the light passed through.

I should mention that some of the sextants that come up on ebay have been in 
storage for as much as twenty years. I bought one recently that appeared to 
be absolutely brand new, with the original packing materials in the case, 
that was manufactured in 1976. That's a great thing EXCEPT that sometimes the 
lubricants have dried or otherwise changed where they're exposed to air, and 
parts may be "frozen". In particular, the housings that hold the rotating 
polarizer in sextants may seize up. A little gentle prying with a very thing 
screwdriver blade got mine moving again.

-FER



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