In addition to Selectric symbol balls, you could use a "normal" typewriter with a set of extra symbols on tiny plastic sticks. You would place the symbol head on the stick in the way of the typewriter's strike.
Indeed, pre-TeX, Phyllis Winkler prepared all of Knuth's papers using TYPIT sticks for all the math symbols. Unfortunately, I didn't nab her typewriter and TYPIT box after her retirement, but here's a good set of photos and explanation: https://twitter.com/mwichary/status/1098850604640755712?lang...
I found a Math Overflow thread full of stories about this: https://mathoverflow.net/questions/19930/writing-papers-in-p...
And see page 7 of this for pictures: https://etconline.org/backissues/ETC099.pdf