Per Ron Fournier (master fabricator and Penske team member) in the Michael Argetsinger book, Mark Donohue: His Life in Photographs,
"Mark asked me to do something about the air that was packing up in the front fenders and causing front-end lift. Those spaces in the door and into the rear body fender were to allow the air to disperse. And it worked really well. What you don't see in the photographs is that there were also holes in front of the door and the rear of the front fender. It was not for brake cooling as most people thought, although here may have been some secondary gain in that regard for the rear brakes. When they made us close up the holes we went to a larger chin spoiler adding about a half-inch along the bottom. Mark didn't initially want to do that because of the added drag--that's why we had come up with the door openings...."
Very nicely detailed photos and additional text included in the book noted above.