MLA

Kissinger, Henry, 1923-2023. Madrid, Spain. Box 771, folder 2. 1975 May 23. https://collections.library.yale.edu/catalog/11776588.

APA, 6th edition

Kissinger, Henry, 1923-2023. (["Box 771, folder 2"])(1975 May 23)[Madrid, Spain]. https://collections.library.yale.edu/catalog/11776588.