Where is my key?

Hello,

I played Sins of a solar Empire ages ago. Like when it came out. Now I would like to register it to play it again with all updates. I still have the original box with the DVD case, the poster with the tech trees and the small leaflet with the key shortcuts. I can't find a key anywhere. I wasn't asked for it to install and play it again.

Is there a way to receive it? Or did I lose something? I can't remember what was all in the box.

Thanks for help ahead!