I don't know what that is off the top of my head. Try looking in some of the Mustang restoration parts places catalogs and see if there is a picture of it near the vent window parts. Sorry, but thats all the help I can offer.
That looks like one of the window stops that keeps the window from going too far down into the door. It has been manymanymany years since I have cannibalized, worked on and upgraded many Mustangs, but I believe it is a window stop.
(edit) I believe it bolts to the front end of each door window. It could also be a guide/stop.
Correct. Window stop. THat one goes on the front metal frame ofthe door glass, right near where the front of the glass rides in the felt channel. You will need this to properly align the window and get a tight uniform fit in the roof rail seal.