Amsterdam, 23 May 1956 - Institutional correspondence Willem Sandberg's letter to Bo Wennberg, dated 23 May 1956
Stockholm, 7 June 1956 - Institutional correspondence Bo Wennberg's letter to Willem Sandberg, dated 7 June 1956
Amsterdam, 28 January 1957 - Institutional correspondence Willem Sandberg's letter to Bo Wennberg, dated 28 January 1957