On Skip's Vintage Trek site he says some interesting things about these codes:
http://www.vintage-trek.com/component_dates.htm#shimano


5. "On dates which can be read backwards (upside down) Shimano underscores the date code."
7. "Pre-76 hubs appear to have a single year letter code (without the month designator). A hub I have has a Y which I assume is 1974, which matches my estimated age for the hubs."

5) so far my wild theory
7) If he is right on point 7 it explains your code. But what about the H in that case...