A quick search on Google on the 4.2 inch mortar served up this page:
http://www.4point2.org/mortar42.htm
As you can see, the range increased several times over WW2 from the section "Increasing the Range in World War II"
So most likely the HE ammo was M1, M2 and so on and the OOB designer has designated the
weapons with the ammo's M numbers and not the ordnance's M number in order to differentiate these, rather than using (MR), (LR) or such a scheme - is my guess.
("Mortar, 4.2in M2 with Shell, 4.2 inch HE M2" would not fit the game's name length limits
!)
cheers
Andy