NavList:

One more thought on the back-to-front scope in the sextant:  I checked IMDB and see that the historical advisor to the series was Robin Craig.  He was a noted maritime historian with commerical maritime experience before his academic work.  Perhaps (I speculate) the use of a sight-tube instead of a scope during a scene depicting a lesson with a sextant was just the sort of subtlety we should expect.

I'll bet nobody would have imagined that a fringe group of navigation aficionados (us) would be discussingit 50 years later :-).