Amsterdam, 3 December 1955 - Institutional correspondence Willem Sandberg's letter to Dora Maar, dated 3 December 1955
Amsterdam, 16 December 1955 - Institutional correspondence Willem Sandberg's letter to Dora Maar, dated 16 December 1955
Amsterdam, 27 December 1955 - Institutional correspondence Willem Sandberg's letter to l'Association Française d'Action Artistique