They were bolted on at the dealer. I believe it was 57 for the first factory 4x4, and it was NAPCO. Awesome trucks.
Yes, '57 was the first year for a factory install of the 4x4 system on Chevrolet, though it was still manufactured by Napco. GMC offered it in '56, same components, plus the GMC could be had with V8 and automatic trans, first in a 4x4 of any make (I can hear the Jeep guys whining now

). Prior to te '55 ist, the Napco conversions were 3/4 and 1 ton only.
In addition to the Napco, American Coleman also did conversions, though they are mostly on 1 ton and larger.