Most European 12 volt cars back then maintained the negative side as the hot side, not negative ground like American cars switch to when they converted to 12 volt. GM converted Buick, Cadillac and Oldsmobiles (SP?)to 12V in 1953,not sure about Pontiac and they converted Cheverolet to 12V in 1955.
Mopar and Ford switch all their cars to 12V in 1956.
All the current in battery flows from the negative side back to the positive side, they only flow from positive to negative internally
Hence the importance of proper and good grounding
Both ways work