my 78 has sliding rear glass. I was under the impression that it was installed by a camper shell company. The camper shell company installed the shell and insulated the truck bed.
Same here mine has a factory option sliding window.. im actually looking for a straight flat back glass.
I know for some of the newer trucks the sliding back window was an upgrade from the rear glass on the "higher" trims. Maybe it was an option after 75 when they introduced the SR5 trim
Just spoke to my friend's dad who used to install the beds and windows on these pickups when they came into port in California. he said for some trucks, they'd pull out the one piece glass, throw it away, and then put the sliding glass window on as a factory option. He said he also used to install the AC units as a factory option.