I finally read the New Yorker article on Susan Kare, ‘The Woman Who Gave the Macintosh a Smile’. And loved this detail about the Command icon on every Mac keyboard:

The command icon, still right there to the left of your space bar, was based on a Swedish campground sign meaning “interesting feature,” pulled from a book of historical symbols.

The hardest icons were actions that didn’t have a real world equivalent, like Undo. And what about the Copy icon?

At one point, there was to be an icon of a copy machine for making a copy of a file, and users would drag and drop a file onto it to copy it, but it was difficult to render a copier at that scale. Kare also tried a cat in a mirror, for copycat. Neither made the cut.