MLA

H. D. (Hilda Doolittle), 1886-1961. Heydt, Erich. Box 11, folder 373-75. 1954-61, n.d.. https://collections.library.yale.edu/catalog/2065204.

APA, 6th edition

H. D. (Hilda Doolittle), 1886-1961. (["Box 11, folder 373-75"])(1954-61, n.d.)[Heydt, Erich]. https://collections.library.yale.edu/catalog/2065204.