On cars that recieved the black tutone hood paint, the trailing edge of the underside of the hood was also painted black from the rear edge, about 5 or 6 inches deep. The sides were masked straight, but the leading edge ( horizontal to the back edge of the hood) was unmasked, creating a sloppy overspray effect.
Mine had this from the factory, and I have seen it on every factory-painted tutone black ram-air hood I have ever seen.
I have seen several factory argent ( silver) painted tutone hoods, and NONE of them had the same treatment.
I have never heard an "official" explanation for why this was done, but I have a guess:
The rear underside of the ramair hood is quite visible from inside the car. If the hood has black tutone, there is a very eye-catching difference in the paint between the top and bottom side while driving/ sitting in the car. To draw attention away from that area, the black was extended on the bottomside a few inches to try and make this not as noticeable.
On an argent-painted hood, the effect would be reversed, since the argent is lighter-toned than the body color. Painting the underside on an argent hood would actually draw your eye towards that, so...on argent hoods the underside was left unpainted.
This is all, of course just a wild-ass guess on my part.
It would be interesting to find out the "official" reason.