The Mortier de 58 T N°1 bis sometimes referred to as Lance Torpilles was an early French medium trench mortar.

For infantry advancing across no man's land, all they may see is a small horizontal opening at waist level, with just the top of the gun shield showing.

[3] The problem for the French Army was they lacked light, portable, simple, and inexpensive firepower that could be brought with them to overcome enemy machine gun nests and wire entanglements.

[4] In 1914, Major Duchêne of the Engineers (of the 33rd Corps belonging to X Army) began experimenting with a simple improvised tube mortar made from discarded Canon de 75 mle 1897 cases at the end of a pole.

He found he was able to build a simple high-explosive fragmentation projectile that could be used as an anti-personnel weapon and to clear barbed wire entanglements.

[5] The basic specification was for a light, mobile, and inexpensive mortar which could fire a 10 kg (22 lb) projectile to a range of 200 m (220 yd) using a smokeless powder propellant charge.

Both designs were much more stable which led to a higher rate of fire because they didn't need as much setup time between shots.

The advantage of the Mortier de 58 T N°2 was that it could fire new heavier projectiles of similar design to a greater range but it weighed 410 kg (900 lb) so it wasn't as mobile.