Ther weren't made factory be GM until '73. Anything earlier was an aftermarket conversion. I do thnk that GM may have provided chassis and body parts directly to the conversion companies for manufacture, and some may have been sold through regulat Chevy/GMC dealers. Most of these conversions were special ordred by construction/logging/railroad companies.
I believe that IHC was the first to build factory crew cabs in the mid-'50s.
