According to my Ford Master Parts Catalog (Final Issue, May, 1975), the "A" servo is used in the HiPo C4 in the 1967 model year (with a "C" used in 1964-66, and an "H" used in 1968).
I am fairly confident this is true, as in the early 70's I bought a bunch of "C" servo sets, and I specifically recall selling several to a friend who had a very original '67 GT350 that had an "A" servo in it.
The exterior tag is the first thing to go when somebody rebuilds a C4. If you are lucky, MAYBE they did not scrap the tag on the valve body. According to my research, a "Green Dot" HiPo valve body will have a "1U" tag on it (verified by several original HiPo C4's I've taken apart). A '67 HiPo C4 valve body will have a tag with "N3", "2N' or "3H" on it. If your valve body has one of these tags on it, then it is likely to be an original HiPo.
You could also remove the tailhousing and pull the governor. The correct governor assembly will have a white paint stripe identifying mark on it, and, when you take the secondary counterweight out of it, you will see that the counterweight is "drilled" on both ends - the large AND the small diameter. Most rebuilders would not change or modify this part, so it is probably a good spot to check if your tranny is an original HiPo C4 or not.
Jeff Speegle is correct that in many cases, rebuilders would just swap in an already rebuilt core instead of actually rebuilding your HiPo C4 - most of them didn't realize there was a difference. That's why I always stood over and WATCHED my HiPo C4's get rebuilt. He's probably also right about judges looking for a "C" servo on a '67 GT350, even though I'm pretty sure they SHOULD be looking for an "A" servo cover.
regards,
Jeff Burgy
SAAC GT350 Tech Exchange Editor