I took the glass out of a 89 D150 which had sliding rear glass & the thick weather striping and I have another sliding glass that seems alittle different, which one is the factory & which is not? heres some pix

another

heres the weatherstripping

If that tinted (smoked) one is in the glass and not film, it is aftermarket.

I am pretty sure the factory ones were never black (dark) tinted, I think they were all 4 pane, also.

Mine had a factory one in it when I got it, but I took it out and tossed it.

More than one company made them

well the one that was used with that thick weatherstripping has a square like frame all around the glass, while the other one has a thick frame with a thin lip that's more shaped like a solid back glass all around with it I guess it uses the thin weatherstripping right?

Mine doesn't use that ugly gasket that sticks out like that and the glass has pentastars on it. But it's a 79 so maybe things changed in the 10 years.
One thing that's a dead giveaway for aftermarket silders is the usually the frame doesn't match the shape of the window opening properly.

I thought that if it was factory glass that it had a pentastar in the corner somewhere?

I found the penastar on one corner which is the one that happen to be busted out by the last owners kids when he had it seems I got 3 after market sliders that use the small weather stripping