Just to narrow the possibilities, here are the TWS stories that precede Merwin's statement: Summer 1945 ("The World Thinker"): Dec. 1946 ("Phalid's Fate"); Aug. 1950 ("New Bodies for Old)'; Feb. 1951 ("Overlords of Maxus"). I've had a quick look at these and don't see a "Meet-the-Author" department in the tables of contents, though there are "Story Behind the Story" features. I'll have to look again after dinner.