[Lynn Eades]

The Ford 3 brush type with the external cut out is 6 volt and I know little about it.
The lucas unit (they all have lucas on the case with the model number and year / month of production)
They are out there in 6 and 12 volt.
If you find a 6 volt lucas, to suit the carrier motor, then you can convert it to 12 volt by changing the field coils in, from a 12 volt lucas unit
A 6 volt amature will happily run 12 volts, but a 12 volt armature wont last in a 6 volt system (I cant substantiate this)
The lucas C39 and C40 units werte made for many years, but the war time ones had a small bakelite terminal block on the side of the case.
By the way, they will ll produce 50 plus volts un regulated (that is part of test)but they wont last long doing it.
Most important; You have to polarise the field windings before turning the key on, when you have fitted a generator. they can be used either negative or positive earth.