[Woodz428]

I don't recall any PI engines that came w/aluminum heads. The F that is the 3rd figure makes me think that they were used in the GT40 program on 427s. I'd have to see a service bulletin like described before I'd buy that story. They probably have much value for someone that is redoing or owns and original GT40MKII or IV. But they are old technology and newer aftermarket heads will most likely out perform them. They used smaller valves than the cast iron heads because of the area needed for hardened seats.
You may be confusing the limited # of aluminum intakes that came on some early PI engines.