Not sure Bob, could be venting from passenger compartment :idonno The Mustangs had problems with air pressure build up in passenger compartment, that I do know. Also could be they are trying to get more air to the brakes.
Here is some nice pictures of a restored one. Holes in the doors also
http://canepa.com/inventory/racecar_1/Penske%20Camaro/index.html
Holes in the doors, holes in the door jam and holes in the wheel well but where does the air come from to cool the brakes? Love how they have the original door panels still in place. :coolio