The one at the very bottom, Minus Six, provided access to the part of the cylinder below the piston, known as the combustion chamber, where a natural gas/air mix was ignited to drive the piston upward on a column of fire. The hatch at Minus Four was above the piston, at least when the gun wasn’t operating. It provided access to the so-called pump chamber, normally full of hydrogen, that got pressurized by the piston when it shot upward.