Hello Steve, I'll take a guess at this. I think the dial is really a heliochronometer with an analemma, not an analemmatic dial. I think the screws up top held a focusing lens or a pinhole aperture that projected a beam on to an analemma on to the lower plate. The analemma is not visible in that picture. The dial is turned to make the beam align, so the hours go counter-clockwise, and the time is read across from stationary indicator at the very top of the dial, hidden from view in this photo. Similar to the upper left dial seen at https://equation-of-time.info/sundials-with-shaped-alidades . -Bill
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