Perhaps you are researching about Iwojima. This unit is usually written as 中迫撃第2大隊 or 中迫撃砲第2大隊. It comes from their main weapon, 10 type-96 150mm (medium) mortars.
Type 96 150 mm Infantry Mortarhttp://en.wikipedia.org/wiki/Type_96_15 ... try_Mortar
This weapon was originally intended for poison gas warfare. In the last stage of war, mortars of various type reserved for this kind of war served as the last reinforcement to poorly equipped troops.
This is a translated article of a Japanese newspaper. Some information had dropped on translation. In original article, the fallen NCO Tamura belonged to the 2nd medium trench mortar battalion, 109th division. He was recalled in 1944 and was dead at the age of 39. So this troop contained aged, reserve personnel. I guess this battalion mainly consisted of aged reserve personnel and was hastily raised to enhance Iwojima.http://worldwartwozone.com/forums/index ... is-family/
Once I explained general situation of Kuribayashi's 109th patchwork division in another thread.viewtopic.php?f=65&t=91386&p=1482838#p812273
This unit was originally a GHQ unit and the number was unchanged after attached to 109th division. 2nd and 3rd went to 109th division and 4th-7th served in Philippine. I don't know why they skipped 1st.