All the sliding windows that I've seen over the years that reuse the non sliding window factory gasket are after market replacement windows. A good indication that you may have a after market replacement window is they stick out away from the back of the cab. The reason for that is the after market replacement sliding windows have a rim around the center on the frame that fits in the factory non slider gasket that is the same thickness as the standard non sliding glass windows.
All the factory sliding windows that I've seen fit flush against the back of the cab and use a wider black rubber gasket. They were this way from 1973-1983 and the frames were gray aluminum color during these model years.
1984 the factory sliders switched to black frames and used a black rubber seal with a chrome strip in the center that matched the chrome strip like the one that surrounded the windshield gasket on the upper trim levels. Lower trim levels just got a black strip in the center of the gasket in 1984 -1987 IIRC.
