You can get a manual here
Info on how to get the password
http://www.royalsignals.org.uk/#info
And I think this is one of the manuals you need
373 Generating Sets DC 300-W 15-V B 136, Johnson Chore Horse, Identification List, EMER, Jul 1948
(under the Batteries, Chargers, Generators, Inverters and Power Supplies heading)
It's generally a great site for manuals!
