Parece que o número de série está no arquivo SetupInfo.plist , mas é ofuscado de alguma forma.
EDIT: O texto a seguir é de aqui :
Is there an easy way to do trial and error and change the key if it was already used?
If you look in the Application menu for the About item and display that, it will display the Product ID to you.
If you find two machines on which all but the last five digits match, the same key was used on those two machines.
It won't tell you "which" key was used, just that the "same" key was used.
Only trial and error will let you sort out which key belongs to which computer. You can change the licenses without removing and reinstalling however.
Easiest method: Launch the Remove Office tool and hold down Option at the first dialog, and the "remove office" option will change to "remove licensing information".
/Application/Microsoft Office 2008/Additional Tools/Remove Office
If you have the 3 systems connected, start one of the Office apps on the oldest of the bunch (IIRC the product keys are in a list so you probably used the top one on that system.). Then launch the same app on one of the other two. If there is a conflict modify the second one's product key. If not the 3rd one must be the odd man out - although you may still have to get 2 & 3 to bump heads in order to be certain.