[Arran]

 I think that Terry figured out what it is, it's a B+ or anode supply with a dynamotor, probably for military electronic equipment, such as a radio, in a tank or maybe a "portable" base station. It does roughly what a vibrator type power supply does in a car radio, except it produces more voltage and more current, 6 volts goes in from a storage battery, and several hundred volts come out. I see that near the commutator there is a brass plate with some other information on it, that may answer some questions.
Regards
Arran