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It is possible that set came from the factory as a 60Hz model and was converted to 25Hz by its owner.
Niagaras 25Hz was IIRC among the first AC plants. Niagara was originally going to be a compressed air plant and the turbines etc were built before the decision to make electricity... the easiest frequency to use to work with the mechanics was 25Hz and there was no other AC around to worry about compatibility with so that is what they did.
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