Re: How to identify an unknown transformer.
The dimensions of the lamination stack should give you a rough idea of the VA rating. The windings' DC resistances should separate the lower voltage ones from the higher voltage (although in a large transformer the mains primary might have quite a low resistance). Then it's a matter of applying a very few AC volts to the lowest resistance winding and, with care, measuring the open circuit voltages on all the others. If you can pick out the 6.3V winding then you might be able to work out which is the primary from there.
Cheers,
GJ
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