One guy in the linked article proposed that one of the two barrels might hold a propellant charge, and the knob on top might adjust how much of an opening was made, to control the range of the projectile.
It might affix on to an established bracket, to target a fixed reference point, from which observers could more accurately adjust the fire of their artillery/mortars.
Spotting gun/range finder?
I was thinking along that line, but with the notion that the top cylinder was nitrogen to extinguish and clear any remaining phosphorus fragments and cool the lower barrel.