They were made at the factory in 56 for GMC and 57 for chevy.
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Apparently NAPCO reached an agreement with both GMC and Chevrolet to supply them with the Powr-Pak conversion kits, and GMC began to produce 4x4 trucks on the factory assembly line (using NAPCO components) starting in 1956, with Chevrolet following suite in 1957.
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