HD,
Maybe the Wide pocket on the case had a belt looped/snapped thru it, like the belt part of the harness. The Top Loop of the case was probably attached to the parachute suspender harness. And the bottom lashing of the case went around the upper knee/lower thigh or below the knee on the upper calve........ or a tie off for the rucksack to hang from when released (?). So once you cleared yourself of the chute, the gun was clear to get to quickly.

Here's a pic of a decent priced repro much like your real one:

WWII Airborne Paratrooper Rifle Case, Reproduction - WWII Soldier

I know........... KEEP LOOKING

CH-P777