lower right in graphite: rf Lichtenstein; lower center inscribed in plate: Reproduced from "Knock, Knock" 1961
Gift to NGA, 1996.
- Corlett, Mary Lee and Ruth E. Fine. The Prints of Roy Lichtenstein: A Catalogue Raisonné 1948-1993. New York and Washington: Hudson Hills Press in association with the National Gallery of Art, 1994, 293.