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I dont know the details.

But do you see the wooden"handle" on the right. Once a day you "charge" it. The stone must be something like a contra-weight. Thats all i can make of it :)

It might be working on a spring just like old clocks, I had seen my granfather charging it long time ago.

Yes a spring that must be it. Cool, my grandparrents had a similar clock-system. Only smaller or course. Simple but effective.

Peace