The SunTour SX-100 is the SunTour S-1 redesigned so that it fits to a drop-out and does not need any special fittings on the chainstay. The SX-100 also has the indexing in the lever (usually a twist grip), not in the derailleur. The pulley cage is very short, as this gear is aimed at commuter and utility bikes with a single chainwheel.
It retains the somewhat elegant the horizontal parallelogram, but the mounting has become gross. This example has a Japanese-style ‘hanger’ with a rear facing slot.