If you want to know how far into the 50s that 6v positive ground systems were available on GMC trucks,
check out the Sales Brochures on this web site. There really was not a "2nd series" for 1955 GMC trucks, there was the Z-series (1954/55 models) and then a new series in late 1955. They were call "Blue Chip" but I think that also had a series letter-designation.
If you look at the brochures, you'll see that 6v positive ground was available standard and as an option in different years/sizes of Blue Chip GMCs.
Tim
