I have finally figured out what this oddball truck is that was included in the photo sets I received:
Turns out, the answer was buried here on the board all along in
this thread. It's a Brazilian spec C10!
Apparently, Chevrolet commissioned a Brazilian company to assemble the trucks for them in South America, and they used many carry over parts from previous generations of pickups. The year ranges for the specific body styles were also different.
This site details the specs for the South American C10s by year.