New York, 9 September 1981 - Institutional correspondence An internal letter from Richard E. Oldenburg to staff at the Museum of Modern Art, New York
Zurich, 12 December 1955 - Institutional correspondence A letter from Klaus Herdeg to Willem Sandberg
New York, 24 March 1980 - Institutional correspondence A letter from Richard Oldenburg and William Rubin to Javier Tusell