Paris, 29 September 1955 - Institutional correspondence Daniel-Henry Kahnweiler's letter to Willem Sandberg, dated 29 September 1955
Amsterdam, 23 January 1956 - Institutional correspondence Willem Sandberg's letter to Alexander Calder, dated 23 January 1956
Amsterdam, 22 May 1956 - Institutional correspondence Willem Sandberg's letter to Pablo Picasso, dated 22 May 1956