New York, 12 March 1956 - Institutional correspondence Bernard Karpel's letter to Willem Sandberg, dated 12 March 1956
New York, 27 November 1956 - Institutional correspondence Bernard Karpel's letter to Willem Sandberg, dated 27 November 1956
Amsterdam, 6 July 1956 - Institutional correspondence A letter from J. M. Trias de Bes to Alberto Martín-Artajo Álvarez