While not exactly a softlight reflector as in the beauty dish, bounce lighting itself was first used in 1956 by Subrata Mitra, but in cinema.
The technique itself was devised to overcome difficulties with exposure that were encountered while filming Aparajito, which is the second amongst the three films of the famous Apu Trilogy by Satyajit Ray.
If there one considers this, and from sanmai's research, we can narrow down the window of this invention to the two decades between 1956 and 1975.
Not sure if this qualifies as an appropriate answer, but I hope this helps!