Waaay back when they thought current flowed in one direction, so everything was positive ground. Later they discovered it flowed in another direction - hence negative ground. At least that's one solution that has been offered ....!
All the older AC's - up through my 1963 D15 Series II at least (which by the way is 12 volt) - were positive ground. You can change it to negative ground if you like if you (a) polarize the generator, (b) reverse the leads on the ammeter, and (c) reverse the wires on the coil. No real reason to do this - 6 volt positive ground works fine.
