New York, 12 March 1956 - Institutional correspondence Bernard Karpel's letter to Willem Sandberg, dated 12 March 1956
New York, 27 November 1956 - Institutional correspondence Bernard Karpel's letter to Willem Sandberg, dated 27 November 1956
28 February 1974 - Press release and communication The artist Tony Shafrazi has freed Guernica from its chains of ownership and has returned its revolutionary nature