Amsterdam, 16 November 1953 - Institutional correspondence Willem Sandberg's letter to Pablo Picasso, dated 16 November 1953
Amsterdam, 22 May 1956 - Institutional correspondence Willem Sandberg's letter to Pablo Picasso, dated 22 May 1956
19 July 1956 - Personal correspondence Daniel-Henry Kahnweiler's letter to Pablo Picasso, dated 19 July 1956