NavList:

Frank Reed you wrote “It also appears to me that the frame for the horizon mirror is mounted backwards. “
I agree.  In that position, the horizon mirror, if it were there, would end up facing forwards, not backwards.  The horizon mirror frame needs turning 180 degrees clockwise or anticlockwise, but how is it attached?  Are there screws on the rear we can’t see, or is it silver soldered into a slot in the main frame?  My Hughes Three Circle uses a screw on the rear and a single locator dowel, so it would be impossible to fit upside down, maybe the Hezzanith has two locator dowels.
You also wrote “the handle looks like it may have been grafted on from another old sextant. I would expect something in turned, polished wood for the original handle...”
The handle looks the same as the one on the eBay example, except it’s been stripped of its original polish or varnish.  I get the impression that Art’s Hezzanith has been carefully stripped down with a view to restoration, and then hastily thrown together, possibly by a different person with the jar or tobacco tin holding the mirrors and clips being lost or overlooked in the process.
Re the clips, they must have been available in boxes of 100 in the days when every port had a sextant repairer.  My 1941 Hughes Mates 6” Three Circle uses the same clips.  Today they must be rarer than 3V bulbs for Hughes bubble sextants, but there might be the remnants of a box in a cupboard somewhere.
As for screws, a spectacle wearer might come up with a couple to fit by going through their cupboardful of old spectacles put to one side ‘just in case the new ones didn’t work'.  Failing that, a visit to the local specs shop might be successful.  Avoid the receptionist and the optician, look for the technician who sits with a tray of assorted screws in front of them dealing with customers coming in for repairs to their unbelievably expensive frames.  DaveP