Amsterdam, 2 December 1955 - Personal correspondence Willem Sandberg's letter to Maurice Lefebvre-Foinet, dated 2 December 1955
Amsterdam, 9 December 1955 - Personal correspondence Willem Sandberg's letter to Maurice Lefebvre-Foinet, dated 9 December 1955
19 July 1956 - Personal correspondence Daniel-Henry Kahnweiler's letter to Pablo Picasso, dated 19 July 1956