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The photos are rather low resolution, but I don't think those are mirrors on
the back side. They look like "mounting points" of some sort (if so then the
"frame" was relatively mobile while the "arm" would have been fixed in
position). Perhaps this device was mounted on an artillery piece for
measuring angle of elevation (?).

You might consider asking the seller for a better photo of the scale, or if
they're not skilled with the digital camera (which seems likely from the
photo quality) maybe you could ask for a description of the markings on the
scale. If it's a true reflecting instrument, like a navigator's sextant or
octant, then the angles marked on the scale would be twice the angle
measured at the center of rotation. If instead it's just a fancy protractor
with a sighting tube, then the angle on the scale would be the same as the
angle at the center. Second, is there a vernier scale on the metal plate
attached to the arm? If not, then the device was probably only intended to
measure angles to the nearest degree or so.

 -FER
www.HistoricalAtlas.com

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