MLA

Luhan, Mabel Dodge, 1879-1962. Cather, Willa. Box 6, folder 174-176. 1923-42, n.d.. https://collections.library.yale.edu/catalog/16708767.

APA, 6th edition

Luhan, Mabel Dodge, 1879-1962. (["Box 6, folder 174-176"])(1923-42, n.d.)[Cather, Willa]. https://collections.library.yale.edu/catalog/16708767.