The 81 mm mortar is one of the most special weapons of the Maginot Line. Unlike most other weapons it has a fixed elevation of 45°. To change the range of the weapon a small triangular had been cut out near the breech. This was linked to a rectangular valve. By turning the barrel the size of the opening could be changed. This way the gunners could let a part of the powder gasses escape, thereby changing the velocity and the range of the shell. These gasses were collected in two cilinders on top of the mortar. This complicated system was choosen to reduce the size of the embrasure. Like all fortress weapons it is breech loaded, the breech being a modified De Bange type. For more information click here.