The LMC Truck parts catalogs are FREE and are the most detailed, fully illustrated and accurate truck parts information source available anywhere These truck
Ok, I've made a little progress, just found a generic diagram of the situation in an LMC Truck Parts catalog. It appears that part (number 7 in the diagram) is called the "window channel".
LMC Truck carries parts catalogs for pickup trucks, SUVs and vans. It carries part catalogs for many popular car manufacturers, including GMC, Ford, Chevrolet and Dodge. LMC Truck states the catalogs are detailed, accurate and fully illustrated.