Robert Nelson first started featuring the "Secret Invention", as early as 1939, and later models were named, the "NEW Secret Invention". It IS likely, that if BIll Dodson didn't make the first ones, he did make most of them, certainly the best ones.
Please remember, while the "S.I" looks cumberson today, it was the most advanced in radio communication then. Also in the 1930's and 40's, Nelson sold or leased them, for $599.00! You could buy a buzz saw illusion or a new car with that money back then...You had to have a serious reason to use it, unless you were a hobbyist/collector with money to spend.
The "S.I" was used for 2-way communication for mental acts, horoscope pitchers, psychic work. Nelson even hinted at it's value in listening covertly to conversations or in card games.(!)
Nelson was constantly beating the drum, about how much MONEY could be made using it, as he did all his products. Several models of the "S.I" came out, over the years.
By the 1960's as transistors came out, he sold a lower priced product, called, "The Mental Mic", and according to one former employee, he remembered maybe selling only one or two a year, by then, as 2-person mental acts had become mostly passe'.
He later sold "Psycho the Telepathic Brain", that did give a silent impulse, for coding/guiding the person on the receiving end. T.A. Waters told me those were partially made from parts used for remote-control model airplanes.
Despite the high prices they originally sold for, they have gone down in price, the rare times I've have seen them sold. I asked the pricest magic collectable dealer a few years ago what he thought they would bring, and he said $100-150. Hopefully they will go up, as I have 3 different models.
The "S.I." did come with an instruction manual, giving details in it's use and upkeep.
As stated in a thread years ago, most cell phones can do what the "S.I" and "Psycho" did, for much less money, and much smaller besides.