[robinhouston]

I don't know, and I'd love to.

If I had to guess, I'd guess Henry Dreyfuss's Symbol Sourcebook. It was published in 1972, and it seems plausibly the sort of book someone like Susan Kate might have had to hand in the early '80s. https://www.societyofsigns.com/projects/symbol-sourcebook

[wsh]

Symbol Sourcebook would’ve been my first guess, too, but I just glanced through my copy (7th printing, 1977) and didn’t see the ⌘ symbol. The closest thing in the Graphic Form Section is a symbol for “Atomic d orbital,” but it’s clearly not the same one that inspired Susan Kare.

[macintux]

Around 15:30 in this video she talks about it, and there’s a slide showing other symbols that may or may not be from the same book.

https://vimeo.com/151277875

[wsh]

Interesting. The left side of the slide at 15:43 in the video is definitely from page 27 of Symbol Sourcebook, but the detail of the ⌘ symbol doesn’t seem to be: not only could I not find the symbol, but also its caption (“FEATURE”) is set in Helvetica rather than Univers as used in the book.

[macintux]

I have a suspicion that she may no longer possess or even remember the book in question. Heaven knows I wouldn’t were I her, but my memory is atrocious.

[Vespasian]

Does anybody know of a modern day equivalent in the form of a searchable symbol database maybe even with a "freehand drawn" image search?

Unicode does not quite cover it because it lacks context and meaning of combined codepoints.

[ranger207]

It's not for everything (it doesn't even have the symbol in the article), but https://detexify.kirelabs.org/classify.html is useful for a lot of math stuff