From 1942 to 1956 all chevy NAPCO's were aftermarket installs . In 1957 you could RPO right from the dealer order your chevy truck with NAPCO installed. Earlier versions used Dana 24's and 3/4 ton axles like Paul said. In 57 dana 23 was used and the axles were changed to passenger side . Half tons still used Dana 24's . But the 3/4 and 1 tons used the new dana 23 set up.
Earlier AD trucks were converted using the dana 23 system. (unsure of this fact) These conversions were not available for the 1/2 tons.
