There's also casual mention of ISO 9573 on historical comp.text.sgml Usenet archives.
David and other people involved with SGML, MathML, and early entity sets for math (and chemical etc.) symbols are hanging around on the xml-dev mailing list (https://www.xml.org/xml-dev) and perhaps can tell more about the origin of that character (which looks more like a symbol for military or electrotechnical use to my totally uneducated eye).
Also, there's a typo in your post: Belisage Conference -> Balisage Conference ;)
Good luck.