It might be the same as the markers found in these "Yes & Know" books: https://www.amazon.com/Lee-Publications-Invisible-Know-8-88/dp/B000IMT3PS https://www.am

It might be the same as the markers found in these "Yes & Know" books: https://www.amazon.com/Lee-Publications-Invisible-Know-8-88/dp/B000IMT3PS https://www.amazon.com/Lee-Publications-Invisible-Know-8-88/...