Difference between early and later factory rear slider windows

[61CustomCamper]

I'm missing a piece of trim from the factory sliding window in my 75 GMC - I've been trying to find a slider window to pull the missing part from but all the ones I've found so far have thinner glass, and don't fit. One was from a 78 and the other from an 80 - both looked like factory windows but the glass was much thinner. I thought the glass thickness didn't change until 1980 or so - does anyone know when the glass thickness/slider window design changed on these trucks?