Complex sentence: Exact death tolls inflicted by the S-mine are not known; the Allies did not record whether a death was caused by a particular type of weapon, only whether or not the death occurred in the course of battle.
Split into simpler sentences: Exact death tolls inflicted by the S-mine are not known. The Allies did not record whether a death was caused by a particular type of weapon, only whether or not the death occurred in the course of battle.