Thanks all. Perhaps part of the mystery that may be solved, if I can contribute anything based on my furniture finishing experience, is the UC designation. Only a guess, of course, but in the furniture finishing trade we reference UTC, or universal tinting colors. These reference basic colors off a standard chart. My further guesses are that, as others have surmised, HC stands for something particular to House of Color paint manufacturer, and HM to Honda Motors paint color. Until House of Color or Honda Motors wants to give up their recipes, we'll all be shooting in the dark.
I know there are finishers out there who can duplicate these colors without much effort. Maybe we should engage one to decipher the code!