Although Bounce Lighting itself was not a Softlight reflector like the Beauty Dish, it was first used by Subrata Mitra in the cinema in 1956.
The technique itself was developed to overcome the difficulties of exposure that occurred while filming Aparajito. This is the second of the three films of the famous Apu trilogy by Satyajit Ray.
Taking this into consideration, and deducing Sanmai's research, we can limit the window of this invention to the two decades between 1956 and 1975.
I'm not sure if this is an appropriate answer, but I hope that helps!