MLA

Beinecke, Edwin J. (Edwin John), 1886-1970. Mulligan, Kathleen. Box 76, folder 1951. 1938 April 4. https://collections.library.yale.edu/catalog/16609966.

APA, 6th edition

Beinecke, Edwin J. (Edwin John), 1886-1970. (["Box 76, folder 1951"])(1938 April 4)[Mulligan, Kathleen]. https://collections.library.yale.edu/catalog/16609966.