Martin is Martin Gardner of course and the magic is his "The 3-by-3 Matix" which first appeared in Scientific American (August, 1966) and is reprinted in his Martin Gardner Presents, 1993, Kaufman and Greenburg, p.149. Word Ways subscriber Max Maven has written a fine introduction to this book and has himself developed a new version of the effect.
"More Martin Magic,"
Word Ways: Vol. 45
, Article 11.
Available at: https://digitalcommons.butler.edu/wordways/vol45/iss2/11