Thanks!
Makes sense... The DVD's I have are V25.00 rather than 25.10, but no big difference I imagine.
I still don't know what the unlabeled key is, but the machine was definitely running with a "Gametime" menu, not regular Force. Guess I could try 2007.5 and 2006.5 iso's to try to find which version works with the unlabeled key..., assuming GT needed a new key every year just like the commercial units. But, if they weren't ever updated, the April 2005 machine must've been a 2006 edition, and the December 2006 machine a 2007, right?