I would guess from the diagram that there's a whetstone bridge or some sort of rectifier. The thing that's in the middle of the rectifier/whetstone bridge in the diagram, I'd guess is some sort of generator, perhaps a spring wound generator which would explain the wind up key. So, perhaps this is a device you wind up and the spring powers a generator like an old fashion clock and the output is a fixed DC output which could be used to power lights, a radio etc. Sort of a mechanical battery. But I am guessing here.