Office 2004 help please!

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So about two years ago I bought office 2004 for my iMac, I used a product key and forgot about it. Now I need to install in on two other computers and have no idea which out the three that it came with I used. (I have student edition)
Is there any way to tell which number I have used on my iMac?
Thanks,
Josh

 

[Santa Rosa]

If I was to guess you probably used the top one, just some logical thinking.

 

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I did think of this, but I want to know for sure.

I have heard that you can remove just the office product key and then put in another, but if I were to do this would all of the three keys work as I had already used one but then removed it?