XD.
The 400 srs combiplane and the equivalent military 400M srs are convertable cargo or combined cargo-passenger versions of the 200srs. the 600 srs is similar to the 400srs but lacks the reinforced and watertight cargo floor.therefore the 600 srs is more attractive as a commercial cargo aircraft as it is a lighter aircraft, the 500 srs had a larger freight bay and the later style cockpit layout.
Best regards,
om15