Complex sentence: Aft, the fuselage was a flat-faced, rectangular section structure based on four steel longerons and fabric covered externally, though there were internal steel plates in the floor and ceiling, and aluminium walls.
Split into simpler sentences: Aft, the fuselage was a flat-faced, rectangular section structure based on four steel longerons and fabric covered externally. Internally there were steel plates in the floor and ceiling, and aluminium walls.