NavList:

This certainly is strange. It raises even more questions in my mind. First, I don't understand what you mean by" apparently reads zero, 19, 18 - - - down to 10".

Second, the vernier is labeled as if it were to be used with a bi-directional main scale, like that on a surveyor's transit, main scale reading left to right and right to left, but sextants have a uni-directional main scale. Is the vernier simply mislabeled? 

Third, a traditional vernier is divided into N intervals, but occupying the distance subtended by N-1 intervals on the main scale. That is not the case here. The vernier is divided into 20 intervals occupying the distance subtended by 21 intervals on the main scale. Why?

Fourth, Does an octant have the ability to be calibrated? Meaning, does a reading of 42 degrees really mean 42 , or is it actually , say, 41 39' ? I previously owned an octant, but never noticed.  Does the placement of the vernier affixed to the arm set the calibration? Meaning, is the arm set to a known angle and then the vernier drilled and riveted to the arm? I think not, as that is a tough task for a machinist even today.