hey, i have a book called The Complete book of Mustang and it shows the same picture of the 4 door mustang that you showed. The book says, "by january 1963, design studies were beginning to look a lot like the mustang that would ultimately emerge. the license plate says falcon and the fender badges say cougar on this clay mock up, which explores a 4 door sedan body style." So i believe they never actually made the car. they only made a clay model of the car, and they didnt even call it the mustang yet.