Left 8.5 volt battery is automotive generator D.C. output sent to heater rheostat heater switch (shown as variable capacitor) to manually adjust charging output to 6 volt car battery. WWII was raging and this was to assist battery charging control on car system.
Most generators were grounded Field types and rheostat should be from field to ground.
Some generators were F or Field to to main terminal or B connected with rheostat between terminals.
American cars converted from 6 volt to 12 volt system in 1955 and 1956.
