How can I obtain the SP 2010 license key being used on each server in my farm?

I am trying to build an inventory of machine names, ip addresses, etc. and want to add the license key to the list of machine information.

When I go to Central Admin > Upgrade and Migration > Covert License Type, it shows me the name of the license - but not the actual license key value - the value that was at one time entered into the system.

How can the original license key value be determined for any specific sharepoint server?

Thank you