Here are a couple of secondary works plus a few books by former WUO members who aren't Bill Ayers that might be worth checking into for references to the origins of the symbol. To be fair I should mention that while I will link to Worldcat records to help folks find these in libraries, the "people who bought this also bought" feature of that big online bookseller's site was very helpful here - sad to say they do a better job than OCLC with providing a flexible, user-friendly, data rich book search. Life under capitalism.

Memoirs/primary sources:

A couple of secondary sources:

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