The red cylinders are a fire supression system added after the first flight. The beach balls were found in the floats and were thought to have been put there for added Buoyancy in the event of a leak. The manikin about midway back gives a sense of perspective to the size of the cargo area. You can also see from this image the ribs that were used to form the structural portion of the fuselage. Unlike modern aircraft made out of aluminum these wooden structural members protruded well into the cargo area reducing the overall capacity.