What's the deal on the small back windows?

[Ft.ValloniaStreaker]

I had a guy send me to look at a small back window C50 thinking it was a 67. When i got there turns out it was a 70 vin#CE530P177942 that 0 is for a 1970.
Then when I got home I googled C50s and turns out lots of them have a small back windows. I not really up on big trucks so, tell me the deal on these trucks with the little windows. By the way, it's a totally RUST FREE ex fire truck with 7159 miles!!!!! I want to put the cab on a 1/2 ton frame, and a C10 doghouse.So will it work? Pictures will come Monday when I get to work. Thanks for any info on this.