This should explain it. 47-50 both vents, 51 saw the introduction of ventipane (wing windows), GM then eliminated the side vent.
https://www.flickr.com/photos/advance-design_parts_co/albums/72157649468726416
https://www.flickr.com/photos/advance-design_parts_co/12315230903/in/album-72157640522200463/