While you're looking for your order number, you might want to check if you have the true "initial" 1.0 build. The original 2003 release included a secret UFO easter egg. To see it, hover your mouse over the main menu buttons in this order three times: . A clickable UFO will appear, taking you to the developer credits.
Don’t let a missing order number stop you. Either recover it, or simply buy the game again. Some classics are worth every penny twice over.