This is how I imagined it in April 1994 (with remarks in Russian):
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Main idea was to represent ASCII code directly - multi-position slide switch on the side chooses "mode" that will code bits 5 and 6 (choose couple columns from ASCII chart as shown below) then 4 fingers will press combination of the buttons 1 2 3 4 above that will represent bits 3 2 1 0 respectively then thumb will press one of the strobe buttons on the left side - that enters chosen code with 1 or 0 as bit 4 (left column or right column in pair of the columns of ASCII table below):
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And third larger strobe button (lowest one that is marked below as SPECIAL - STROBE 2) will represent special column of codes that partially mapped to ASCII column 0, partially to ASCII column 1 and partially have some frequently used characters from other columns (this strobe will work in any mode so it's like a shortcut):
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It should say every entered code back to the headphones and user will be able to delete wrongly entered character by inserting "backspace" (press button 3 and then strobe 2).
Best to keep getting your eyes checked out from time to time. Several years ago I was told that in 12 to 15 years I would have to face cataracts and the treatments. Guess I am down to 8 years by now and I currently have a troublesome issue with floaters in my right eye.