This is a non-mystery. It looks like this was done with a Selectric typewriter using two typeballs, one of them "Symbols 10", and the other for text.[1] The reason there are two versions of "4" is because you could type something like "(4.10)" with either the regular typeball or the Symbols ball. Both have numbers, parentheses, upper case letters, and a period/decimal point. Switching typeballs takes a few seconds, s…
When typing documents like the ones shown, the typist would count characters and plan the centering and alignment of columns in advance. I can remember marking up manuscripts with pencil before starting to type on a clean piece of paper. If you made mistakes that couldn't be corrected with whiteout or correction tape, you typed the page again.
I took some mathematics classes from Paul Halmos at UC Santa Barbara in the late 1970s. He and his wife invited a group of students to their home for dinner, and his secretary joined us as well. I remember her saying that she enjoyed the challenge of typing mathematical manuscripts even though she didn’t understand the math.