It might be a voltage reducer... One source lists the otter as 12 volt system. using a voltage reducer would permit use of a 6 volt gauge, common to other CMP... More research is needed.
A photo of a current version
https://vintageautogarage.com/electr...RoC1zQQAvD_BwE is attached. They are also referred to as Runtz
https://oldchevytrucks.com/catalogse...esult/?q=runtz