I searched the Comprehensive Latex Symbol list but couldn't find the symbol below. It's on the very last page of Knuth's Selected Papers on Fun & Games, which might mean it's a puzzle or mystery to go along with the rest of the puzzles in the book (pp. 741 in first printing; pp. 742 in second printing). I don't necessarily want to have the puzzle ruined for myself or others, but I've read the book cover to cover and searched high and low on the internet, and I can't really find any clues on how to proceed in finding more information about it. It's listed in the index as a reference to itself. Any hints?
EDIT: The index can be downloaded here: http://web.stanford.edu/group/cslipublications/cslipublications/site/9781575865843-knuth-fg-index.pdf
In the preface (pp. xiv), Knuth writes:
In conclusion, let me close this preface by warmly thanking the KTG society and its fictious leader, whose existence is sort of an enigma that can be deciphered only be carefully pondering the index at the rear of this book. Readers who are able to solve this weird puzzle will find that my eccentric muse was not at all able to resist the temptation to finish off the final preface of this eight-book series by adding some symbolic "book-ends," so that the fun and games would last longer.