Battery Series Device

[Noswal]

A friend of mine has acquired a supercharged '39 Zephyr. I has two 6 volt batteries and some sort of device below the solenoid that lets it start on 12 volts and reverts to 6 volts once it starts. The device went bad and he found a NOS on ebay. He put this thing on and now he has no power at all to the starter. Maybe wired wrong, but I know nothing about them. Can anybody tell me how they are wired? Or, if there is an alternative way to do this? Thanks.
Butch