Even if it can be bought on CO.com, it does not mean it can be used for SDC. If seatcounter or any other means shows Y/H 1 it probably means they are already oversold and are willing to oversell one more ticket (and hope for no-shows or deal with the oversold situation). Regardless of that, the fact that they are willing to sell one more seat, does not necessarily mean that there are seats available, and therefore room for SDC.