As the Shack was a descendant of the Manchester which was designed at a time when the 'Bomber Transport' was popular I'm not surprised Shacks carried troops as passengers.
The description of the canvas seat back fits in with the arrangements for troops and paratroops in the Manchester and Lancaster to go with the 'Numa' matting on the floor to sit on.
Talking of Bomber Transports is there anyone here who remembers the Argosy's Colonial Policing role that saw it capable of carrying (and dropping) 6 1000 lb bombs.