Regarding Mh, here is what Van Dort et als had to say in their book:
"... It darkens Red in both rooster and hen. Meantime it extends Red and restricts the distribution of Black. Mh has therefore a Columbian like action.
It removes Black from the breast of the rooster and hen and causes a kind of black spangling. Mh also gives red on the shoulders, back, and wingbow a deep dark red colour in the rooster. The salmon breats of the Duckwing "(my note that's e+ based)" hen also becomes dark red red in the presence of Mh, as in Red Duckwing."
When she referred to to the "Columbian like action" I think she is referring to idea of restricting black to the ends of the chicken, ie neck and tail areas. Mh restricts the black from the "front end, but it is still present on the "back" end in the RI Red (Remember that RI Reds are eWh based and "normally" have black breast, primaries, main tail and sickles.). Reeder in "An Introduction to Color Forms of the Domestic Fowl" also has a (similar) discussion of the Mh affect for the RI Red and the similar New Hampshire that lists the genes for them.
Did I get that right?
Dave
Edit: As Chris pointed out, this would be for the males. The females, however, do have the black restricted on the breast.