Fredric Wertham, Seduction of the Innocent (New York: Rinehart, 1954), 185. “Some of the ordinary comic books have illustrations revealing crude sexual details if you look at them in a certain way. The shoulder of a man with a red scarf around his neck shows a girl’s nude body.”
The book’s section of b&w illustrations (between pp. 212-3) shows the comic book panel referred to, also helpfully cropping and magnifying it for the obtuse reader.
It originally appeared in Jungle Comics #98, Feb. 1948.