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Jurgen, Don
Yes, I didn’t read the original question fully.  I allowed myself to be side-tracked by the last sentence, and I went straight to Monsieur Vernier.  It’s true, the scale is not a Vernier as we know one, and yet the font and aging suggests both scales are original.  I googled H G Blair and Co, and there are lots of photographs of their sextants.  There were many different styles of frame over the years, which suggests they probably did do a lot of ‘buying in’.  I hoped I’d find a close-up of the lower scale on one example to see if it was similar to one leading to the question, but I was unable to.  Unless there was something magic about the original scale magnifier, the only possible use I could think for the current set up is, come rain or shine, corrosion or muck, so long as one tiny part of the scales can be read comfortably, you could calculate the Hs by subtracting 1/5^th of any readable value on the lower scale from the corresponding angle on the upper scale.  However, this seems a very explanation.  DaveP