Bruce,
You are referring to this:
That's how H-O supplied the kit back in the day although I made this one for my "TurboForce Revival" Bird from a new Corvair unit and a used HEI vac advance. There was also another application that had a boost retard feature, but I don't remember which, but I think Ken Crocie knows. As you mentioned, H-O had to modify the standard distributor vacuum advance for the larger-in-diameter HEI, which is shown in the picture.
Yes, they are quite pricey today, if you can even find one. That's why it makes sense to use the MSD Boost Timing Master (BTM) instead if you don't have the one H-O provided. I'd guess that Joe has one, but I know that Mike does not. But, I have the adapted Corvair unit on mine.
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Craig Hendrickson, the "H" in H-O Racing Specialties.
http://www.OriginalHO.com