MLA

Erdoes, Richard, 1912-2008. Matthiessen, Peter. Box 178, folder 931. 1983 March 6. https://collections.library.yale.edu/catalog/17441919.

APA, 6th edition

Erdoes, Richard, 1912-2008. (["Box 178, folder 931"])(1983 March 6)[Matthiessen, Peter]. https://collections.library.yale.edu/catalog/17441919.