Ok, i've found something (from "Storia Militare", a very good magazine).
100M is for "Bomba
Mina" (roads, railways, light tonelage ships, bridges).
100T is for "Bomba
Torpedine" (heavy targets, structures, medium tonelage ships)
I need to search again for explosives (TNT ecc. ecc..).
M = Mina (italian) = Mine (english)
T = Torpedine (italian) = Torpedo (english)
100 = 100 Kg bomb