No Cd Key For 2004 Century Of Flight Microsoft Flight Simulator 2004 Century Of Flight Product Key?

Microsoft Flight Simulator 2004 Century of Flight Product Key? - no cd key for 2004 century of flight

Why is Microsoft Flight Simulator 2004 is a product key. I updated for FSX and try to sell my 2004, but I do not see a product or a CD-Key anywhere. They come with one or another way to authenticate the installation?