72 Chevy Sliding Rear Window

[paulspickupparts]

Parting this old 72 out and has the sliding rear window. I usually do the earlier trucks so learning on the 67-72 trucks. Can anyone tell me is this is the original rear sliding window or an aftermarket. It does show it as and option on the glovebox, but not to say some Nebraska farmer did not change it out over the years. Rubber gasket is very soft which has me scratching my head. Thanks for your help.