Actually, here's the first Super Famicom proto:


It did have that compatibility "switch" that merely passed through the video from a redesigned Famicom:


And the controller's buttons were marked A B C D at the time:


This change from the horizontal Famicom's button layout did present an ambiguity and probably some players inside Nintendo preferred using the rightmost buttons for the main action buttons (B,A)... take a look at the button options in the Super GameBoy for proof.