underneath the snes 4021's circuit, it says
<i>To make a NES connector, simply ignore the second IC. Please note that button 'Y' above is button 'A' on a NES.</i>
im not sure about the pinout of the 4021 in reference to the snes, but as far as i can tell for the nes...the 'Y' would be the B button and the 'B' would be the A button.
thought id mention it.