I've long suspected some cross-licensing arrangement between Shimano and Simplex. The early Shimano derailleurs (Eagle, Lark) are very similar in design to the Simplex units with a standard parallelogram and springs on both the upper and lower pivots. About the same time Shimano started using a dropped parallelogram on its Crane and Titlist units, Simplex also came out with dropped parallelogram units as well.

But by the time SunTour's patent on the slant parallelogram expired, Simplex was on its last legs. Shimano incorporated the slant parallelogram with its own dual-spring and dropped parallelogram design with the 7400 series derailleurs in 1985. Virtually all modern derailleurs share these features now.