Loc: Goulburn Australia
If there are batteries installed, check which way they a connected if positive to positive or 2 battery posts on each lead it is 12v but positive to negative with a short cable between batteries it is 24v. No batteries possibley makes it a little more difficult, the generator and starter motor OFTEN are stamped 12v or 24v, without this stamping it takes a bit more investigation. Tony
1938 1/2 ton Hope to drive it before I retire
Check the battery cables to see if they go to a series parallel switch as that would supply 24 volts to the starter while the rest of the system remains 12 volt. Remove a lighting bulb as they are usually marked. If 12 volt the starting system could still be 24 volt as noted previously. The voltage regulator should be marked for pos or neg ground.
Did you figure out the answer to your question? I can get you a contact to get you information if you need. Google the Old Dominion Historical Fire Society. Check out the owners photos. There should be a 900 and you should be able to connect with the owner. If not, let me know and I can hook you up. Mike Denbar