Originally Posted by David Hingtgen

The pic doesn't show it wearing N850US in that scheme---reg number can't be seen. It is however known to have worn PH-ZCI at the time, then later was repainted and seen wearing N850US. It is unlikely (but possible) that it wore N850US in triple-red. I doubt they actually have first-hand proof, they probably just looked up the production history like anyone else, and assumed that like every other US and AA plane, it only wore the PH-Zxx reg for temp fight testing, and was delivered to the airline with its "permanent" registry. But N850US/PH-ZCI was an exception to that rule.

::edit:: Actually, it probably is wearing PH-ZCI at the Fokker site picture. How to tell? Because you can't see it! If it was wearing N850US, it should be visible based on where the reg number is placed on a triple-red DC-9 and F-28. Nor is there a US flag, which'd also be there in front of the reg number if it was in full USAir colors/markings. We can see where the "N850US" should be in that pic (and where the flag should be), and they're not there.