Yes, it's simple but there are a lot of speculations. I think the best you can read is this book (https://books.google.it/books?id=2f1aDgAAQBAJ&pg=PA35&redir_esc=y#v=onepage&q=ishango&f=false) and the Wikipedia page (https://en.wikipedia.org/wiki/Ishango_bone)
(I wanted to answer earlier but I hadn't got much time 😅)
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