I have a 64 CSS with the push button automatic transmisision. Bought new in LA. According to Ben ( the CSS site guy and
all around nice guy), mine might be one of the earliest built trucks, possibly even the "Palomino" that was test marketed prior to production. There was an article in a magazine back in 64 where they refered to the truck as the Palomino.
Mine is unusual in that it does not have the side moulding that most 64 CSS trucks have.
Mine also has frame reinforcements called out on the SERT sticker, which are basically steel channel reinforcement welded the bottom of the frame rails between the front and rear suspension points, and a unique engine crossmember. These were factory modifications.
I almost bought a real 426 HPP about 7 years ago. It seemed a bit pricey at the time but I should have bought it.