A couple Heinlein's stories referred to a Renshaw device. From "Assignment in eternity" at https://archive.org/details/assignmentineter0000hein_h0q8/page/n67/m

A couple Heinlein's stories referred to a Renshaw device. From "Assignment in eternity" at https://archive.org/details/assignmentineter0000hein_h0q8/page/n67/mode/2up?q=%22Renshaw%22+%22Robert+Heinlein%22 https://archive.org/details/assignmentineter0000hein_h0q8/pa... : "Around World War II Dr Samuel Renshaw at the Ohio State University was proving that most people are about one-fifth efficient in using their capacities to see, hear, taste, feel, and remember. His research was swallowed in the morass of communist pseudoscience that obtained after World Word III, but after his death, his findings were preserved underground." That Renshaw is the author of "You're Not As Smart As You Could Be". https://en.wikipedia.org/wiki/Samuel_Renshaw https://en.wikipedia.org/wiki/Samuel_Renshaw lists more places where Heinlein cites Renshaw's tachistoscopic training, including in this fan mail response.