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If we did have parallax present, and 10 units on the lower scale were fitted into 19 degrees 50 minutes on the upper scale, we can work as follows.  The smallest mark on the upper scale is 1/6^th degree or 10 minutes of arc.  The smallest division on the lower scale is 1/6^th unit, effectively 10 seconds of arc, so the lower scale helps the observer divide the smallest division on the upper scale into 1/60 ths or 10 second sections.  To read:  note the number of degrees and 10-minute parts on the upper scale from where the zero on the lower scale meets the upper scale.  Then observe which 10 second mark on the lower scale comes nearest a mark on the upper scale.  Add the number of minutes and 10 second bits thus indicated on the lower scale to the degrees and 10 minutes value indicated by lower scale zero on the upper scale. 
Frank
If setting up a dividing engine took anything like as long as setting up the dividing head on a lathe to make my plywood and ABS sextant, wouldn’t they have made each part separately in batches to save effort and consigned each to separate stock bins for use when required.  DaveP