Ray,
I also was thinking along those lines, but I am a bit unsure and unwilling to give out potentially erroneous information...
Didn't Ford market Shelby parts using a 'S' for a parts code? I once (long ago) had a 715cfm Holley that had the long snout Shelby float bowls and it had both the Holley part number as well as a Ford 'Shelby' part number on it C7(something something)-9210-(something something)) Maybe I'm confusing the warehouse letter ('Z' for Ford and 'Y' for Mercury and (maybe) S or was it X for Shelby) I don't see too many of those parts these days... Not to mention that I killed off those brain cells with that knowlege many, many moons ago.

Hmmm, the more I think about it, the more I'm thinking the T-bird route as well. However,....
That 'HM' casting or stamping number makes me think along the lines of Holman-Moody. Over-the-counter Ford Muscle Part maybe? Gee, I have never been much of an older (60s) Ford parts scrounger. Also, I never got much into FEs or small blocks since I deal primarily with Clevelands and 429/460s.
I have a friend who has an extremely rare over-the-counter Ford Muscle Part intake, complete with the rectangular FoMoCo logo on it... Would you believe a Boss 429
dual quad aluminum intake manifold?
Sorry I can't be of more help.