Looking more closely, Id say its a compounded DC generator. the output current will be found across the two green wires inside the unit. The brushes appear to be seriesed with part of the field coils (stator) giving feedback to hold the voltage stable under load. The exciter current is fed into the two black wires.
Chuck it in a lathe and spin it up with a DC voltmeter across the green wires, and feed in a small current from say a car battery into the two blacks. It will generate, and measurements can be taken to identify is parameters.
Another guess ... its a 120 volt DC unit at around 20 amps.... judging by the winding sizes, probably needs around 15 volts at 2 amps exciter current.....