My 1940 1/2 ton pickup had the original seat upholstery in it when I got it, and the cushion and seat back covers were held in place by C-clips that fasten to the seat frame. These clips are not visible when the seats are in place. My 37 1/2 ton has similar C-clips on the seat cushions, which, I believe identifies the cushions as original, as most upholstery shops likely used hog rings when installing replacement covers.
Jim Carter Classic Truck Parts sells the C-clips, Part No. UP220. It lists the clips as fitting 1937-59 trucks, so it's possible the C-clips weren't used on the 36 trucks.