I have a C5ZZ boxed nos just like Bob's image with 12 threads, plus another installed on 65 Mustang GT that is the same.
Also, have a used valve from a swap meet (unknown origin) that is painted black and has 8 threads. It has an interesting attaching bolt with head marking of X.
Also, have a used valve from a swap meet (unknown origin) that is painted black and has 8 threads. It has an interesting attaching bolt with head marking of X.