I've heard both and don't have a clue to which is really the case. The original S/C's were real competition 427 Cobras that Shelby couldn't sell (he made a bunch for homologation purposes) so a brilliant marketing guy cooked up the idea of selling them as S/C's. That's why they had the hood scoop,
oil cooler, roll bar, and more flare to the fenders, as well as the SW 240 fuel pumps in the trunk, a larger gas tank, and many other features that the street 427's didn't have. Only 36 real S/C's were made, if memory serves me correctly.