This gear caused me distress and dismay when it appeared. SunTour seemed to have abandoned the best design in the world (the slant parallelogram) and faithfully copied the 1975 Huret Duopar, a derailleur that I personally detested for its fragility and dishonesty - it promised so much and delivered so little.
The design did have some interesting details. SunTour had adopted a ‘Light Action’ style sprung cable clamp (in imitation of one of Shimano’s better features). They had also used their infamous guide-pulley-with-an-internal -spring design. True to form this example, which is otherwise completely workable, has a totally destroyed guide pulley.
Despite these details, SunTour’s decision to adopt this geometry was like the director of ‘ER’ deciding steal the script of ‘Holby City’.