I've read that Ford made 12 volt systems standard in 1956 but 6 volt systems were an extra cost option. Mainly due to the Ford police car market. They still had six volt radio systems. From "Ford Police Cars" by Edwin J. Sanow.
And I can't find it but I'm wondering if there was difference between U.S. and Canadian built Fords. We did keep the flathead one year longer here in Canada so maybe six volts too?