I remember back in '03--ten years ago--seeing them on TV, Ford made a big deal that they would have three complete GTs ready to take to the event and I thought they had the body style of the production car, the 2005 body but when I googled Centennial celebration Ford GT the Car & Driver story showed one of the test mules that had a slapped together body (one red one didn't even have the complete front end look ), one of three they made just to test mechanical components. So there's three more that may come up for auction someday.
In the Car & Driver story. the only reason I think they had the wrong car pictured is that big magazines have a three month lead time
so maybe pictures weren't available yet so they ran the mule shot.
2005 Ford GT Road Test – Review – Car and Driver
Hot Rod magazine, in November, had the three I remember from TV news with complete as-per-2005 bodywork
100 Years of Ford Motor Company - Hot Rod Magazine
Have any of the Centennial Ford GTs or test mules come up for sale yet? I bet more than one engineer took one of the latter home. When they do come up for auction it will be interesting to see if they are valued at more than the Las Vegas Secret Auction cars because they are in fact, chronologically earlier.