6V vehicles were a hodgepodge of grounding. I don't recall, anymore, "who did what." At one time I think there was some theory about electrolysis
According to this, Packard stayed with POS ground after switching to 12V
Positive Ground Cars
From the article:
"Positive ground vehicles were once fairly common. In the mid 1920's, nearly half of the cars produced had positive ground, including Rolls Royce. The Ford Motor Company used negative ground polarity on the Model-T, then went to positive ground starting with the Model A, and then went back to negative ground in 1956.
Packard, Nash, Hudson, Pierce, and Studebaker were positive-ground until the mid-fifties. All British vehicles imported to the U.S. after World-War-II were positive ground, and many stayed that way until the early seventies.
Positive ground in automobiles was mostly abandoned with the introduction of 12-volt electrical systems in the fifties. 1955 was pretty much the last use of 6-volt electrics in American-made cars. Most cars switched from 6 volt positive ground to 12 volt negative ground together. An exception to this was the 1955 Packard, switching from 6v to 12v but not switching to negative ground."