Amsterdam, 8 November 1955 - Institutional correspondence Willem Sandberg's letter to Josep Lluís Sert
Paris, 30 January 1964 - Institutional correspondence A letter from the Committee of the exhibition España Libre (Free Spain) to Pablo Picasso
Flushing, New York, 23 May 1939 - Institutional correspondence Simon Guggenheim's and Dana C. Backus's form letter to ask for donations
New York, 25 May 1939 - Institutional correspondence Sidney Janis's letter to the Motion Pictures Artists' Committee
New York, 10 June 1939 - Institutional correspondence Evelyn Ahrend's letter to Marcelene Peterson, dated 10 June 1939
Toronto, 21 July 1939 - Institutional correspondence Phyllis Lane's letter to Evelyn Ahrend, dated 21 July 1939
New York, 4 August 1939 - Institutional correspondence Sidney Janis's telegram to the FIne Arts Gallery of San Diego
Roxbury, 22 November 1955 - Institutional correspondence Alexander Calder's letter to Willem Sandberg, dated 22 November 1955
Cambridge, Massachusetts, 8 December 1955 - Institutional correspondence Josep Lluís Sert's letter to Willem Sandberg
Tokyo, 20 November 1962 - Institutional correspondence Akira Jin's letter to Pablo Picasso, dated 20 November 1962
New York, 28 February 1974 - Institutional correspondence A letter from the Guerrilla Art Action Group to Richard Oldenburg
Amsterdam, 8 November 1955 - Institutional correspondence Willem Sandberg's letter to Alexander Calder, dated 8 November 1955
Amsterdam, 30 November 1955 - Institutional correspondence Willem Sandberg's letter to Alexander Calder, dated 30 November 1955
Amsterdam, 3 December 1955 - Institutional correspondence Willem Sandberg's letter to Dora Maar, dated 3 December 1955
Paris, 5 December 1955 - Institutional correspondence Maurice Lefebvre-Foinet's letter to Willem Sandberg