Your comment with more detail about Chevrolet truck NAPCO factory option first offered in the 1957 model year prompted more research on my part. Thanks for the additional info.
Obtained from source articles:
Starting in 1956, NAPCO aka "Powr Pak 4×4" was offered as a factory-installed option on GMCs, while Chevrolet began offering it from the factory in 1957 (assigning it a Regular Production Option number (RPO 690)). The two were identical systems, other than the availability of a V-8 and an automatic transmission on the GMC trucks. The Chevrolet could only be ordered with the 235 six and a four-speed manual transmission. This continued more or less unchanged until the introduction of the redesigned 1960 trucks, when GM installed its own four-wheel-drive system on light trucks.
Some of the above info is already found through Searches within the Stovebolt site. New to me is the Chevrolet factory RPO 690, and that GMC offered NAPCO with its V8 and Hydramatic automatic transmission while Chevrolet did not.
