Kevin Marti may be the only person who could answer this anomily...it may be interesting to see what other options may be on this particular "1 of 1". If the VIN were known, I wonder if it were a very early build, perhaps so early that it pre-dates some of the literature that was sent to the dealers (that Bob posted some copies of earlier).
I actually asked Kevin Marti about this car way back in July 2000 (shortly after the first edition of the book came out):
Me: "Page 13 - The data shows one Acapulco Blue Fastback with a 4A Bench Seat. Is this correct? In didn't think a bench seat was available in Fastback. If it's true, what was the body code for that car - 63C?"
Kevin Marti: "It wasn't available, but this is why if you're the right person, you can get what you want. I don't know if they coded it as 63C."
He didn't elaborate any further...