Reasonably sure, fwiw, that it was not actually manufactured by GM but the crewcab conversion was done by somebody else. Kinda like the 4WD conversions done by Napco (Neapco?) ... GM or whoever makes most of the truck, sends it out to the jobber to convert, then brings it back, paints it and whatever, and sends it to the dealer.
The back doors look suspiciously like cut-down fronts, which is always a dead giveaway. There are a handful of 67-72 crewcabs running around; for the one side they can use a Suburban's rear door, right, but the Burbs in that era only had the one, so they have to get creative for the other side.
It's a nit picky point, but crewcabs manufactured from the get-go by GM started in 1973.
All that said, if you like the 63-66 body style (or whatever years those are) it's a hell of a find... I don't think it's a four grand find, but then I really prefer the aesthetics of the 73-up body style and find anything older just visually clunky.
-- A