NavList:

Alex you wrote:

"Here is an illustration to what I was saying:
an old sextant that looks nice,
but the index mirror is transparent: all silvering is gone.
See the third picture in the second column.
E-bay item 170047453767."

 

Maybe I missed a photo, but isn't that just the transparent side of the horizon glass? 

 

I wouldn't be surprised if the mirrors do need resilvering though.

 

While I'm thinking of it, sextants usually work without serious problems even with mirrors that have lost 10 or 20 percent of the silvering. Since the focus is at infinity, a few blemishes eight inches away don't really cause any problems. I have an octant from c.1830 that works fine despite missing about 20 percent of the reflective material on both mirrors.