MLA

Wharton, Edith, 1862-1937. Gilder, Richard Watson. Box 25, folder 778. 1902-09, n.d.. https://collections.library.yale.edu/catalog/16797068.

APA, 6th edition

Wharton, Edith, 1862-1937. (["Box 25, folder 778"])(1902-09, n.d.)[Gilder, Richard Watson]. https://collections.library.yale.edu/catalog/16797068.