NAPCO stands for Northwestern Auto Parts Company. Yhey were an early converter of GM and other makes of trucks to 4 wheel drive. They provided the axle, transfer case (sourced from Spicer), driveshafts, and suspension components to the dealers for the 4x4 conversion. Most of their conversions were Chevrolet and GMC, though they did do some for Studebacker and Ford. They also provided GM with components after they offered the 4x4 option as a factory installed option in '56 (GMC) and '57 (Chevrolet). That relationship ended in '60, when GM decided to do it all in-house.
Check
here for more info on NAPCO.
A side note, NAPCO was not the only company that did 4x4 conversions. American-Coleman also did conversions using their own axle and t-case designs. You see them more on heavier trucks, like Mack and International, though GM used them as well. You also see A-C axles on farm equipment, mose front assist tractors from the '50s-'80s use their axle. marmon-Herrington was also made 4x4 conversion kits, though most of threirs are mainly Ford use.
