According to the "Mustang Production Guide" by Jim Smart and Jim Haskell, as Bill stated, the first Mustangs built at Dearborn as 1965 models (alternator charging system instead of generators) were built starting August 1st, 1964 and started with VIN number 250000. This car is listed in the registry section of that book. The authors also state that the "often published and believed" transition date for 65 production was August 17, 1964 although they show August 1st as the date that production began. It is my opinion that this may very well be a show unit or promo car. Sounds very cool!
Ken