1972 C20 Deluxe Extended cab?

[calveryc]

My neighbor has a 1972 C20 Deluxe that has a extended cab with back seat. The rear window is a solid window and the back window apparently opens upward. He is going to sell it soon and I was contemplating buying it. It is 4x4 and has a 454 that seems to run well. The frame has no rust but there are some trouble spots on the truck. My question is what is this truck? I looked everywhere and I can't find one that looks like it. Is it a custom job? I'll try to get pics but I don't have my camera at the moment.