I too have been collecting the set for a couple years. I've seen scans of ad backs, but never seen one for sale. Every ad back I've seen a scan of shows the Supplement folded twice: once each way.
Not that anyone asked, but my fav is the Cobb. Plank is up there too, with Matty and Bresnahan and Magee and the team photos and a few others being close behind. So if anyone has a Cobb available ... lemme know!
Rob