While Chevrolet was still using glass, GMC had gone to bags. This is a kit from 1954-55, it is not aftermarket (though different manufacturers were available) these were accessory items available at the dealer parts counter. They could be installed by the customer or dealer personnel.
https://www.flickr.com/photos/advance-design_parts_co/23812917635/in/album-72157631868452754/
https://www.flickr.com/photos/advance-design_parts_co/23786818416/in/album-72157631868452754/
This is a very rare kit, I have never seen another as of yet.