During the 1970s, SunTour's derailleurs were well regarded. They were the best value and many considered them the best performing. The entire line followed the same basic design, with the big difference between models being the weight and price, primarily due to the amount of steel employed.
With all due respect to Top506, the Seven-GT was not all steel. It used an aluminum casting for the upper pivot housing. The all steel model was the Honor. The Seven-GT was generally found on upper, entry level models.