MLA

Miller, Henry, 1891-1980. Durrell, Lawrence. Box 7, folder 78. undated. https://collections.library.yale.edu/catalog/10251564.

APA, 6th edition

Miller, Henry, 1891-1980. (["Box 7, folder 78"])(undated)[Durrell, Lawrence]. https://collections.library.yale.edu/catalog/10251564.