The Shack has a number of "hard points" built into the fuselage floor to which a piece of canvas held by steel cables is fitted, the ends of the cable have a clip which attaches to the floor points. The canvas is used as a back rest. If ferrying troops the crew is reduced to six (from ten). I believe that the normal quoted figure for number of troops carried is up to 33.
Seems that it was quite a common method to move troops in the early days of the Shack.