Ah yes... this is a good one.
That is the S-81SL, aka 81mm supository launcher.
Only 42 made, placed into service in Fall of 43' in the South Pacific.
Took a crew of 7 men, 3 guys to tackle and hold the enemy down, 1 guy to take the enemy's pants off, 1 guy on lube, one guy to incert the applicator unit, and the last guy to pull the lever. The applicator unit is the metal tube in the center.
It turns out that this activity was difficult during combat, but easy during training against other Marines after a few 6 packs of beer.
So the ever resourceful marines found out that if they threw hand grenades at the enemy first, this caused the enemy to dive for cover protecting their heads, but leaving their buttocks exposed. This gave the S-81SL operator ample time to aim and fire at the enemies exposed posterior.
Unfortunately, the S-81SL slow rate of fire made it's usefulness soon obsolete, and it was eventually replaced by BGRC-20, aka BenGay rubbing cream - but that's another story.
DmdNexus