THe 410 came before the 400..
Luciano
Yes, you're right and I'm sure you're right when you say that only one of these was built.
The puzzle car was Chassis No 1517A which was delivered to Gianni Agnelli in May 1960, and is generally regarded as being the first 400 Superamerica.
However, it is described as being first shown at Turin in November 1959, but the car shown there had the deeper grille and corner bumpers and was described as a 410 Superamerica! It does seem that, as you say, it's the same car which was modified to improve its appearance before being handed over to Agnelli 6 months later, as well as being updated to the newer 400 SA specification.
This would explain why there are plenty of current photos of the 400 SA, which of course is still around, but none of the so-called 410 SA which has never been pictured again since it was displayed at the Turin Show in 1959!