User blog comment:Deedlit11/Ordinal Notations I: Up to the Schutte Klammersymbolen/@comment-1605058-20130630091058/@comment-5529393-20130701152151

Actually, what Hurford talks about is exactly the normal Schutte Klammersymbolen. He's using the convention we used in the "My number is bigger!" thread on the xkcd forums, which is to represent each column as a@b. (So it's like "a at the bth place".)