This article is from the Books FAQ, by Evelyn C. Leeper firstname.lastname@example.org with numerous contributions by others.
"The Analytical Language of John Wilkins," in OTHER INQUISITIONS (University
of Texas Press, 1964). It is not, repeat NOT, "Tlon, Uqbar, Orbis Tertius."
I re-read that with a specific eye toward looking for this reference and it
isn't there. Nor does it appear to be in any other story in LABYRINTHS.