MLA

Barrie, J. M. (James Matthew), 1860-1937. Duncan, Elsie. Box 6, folder 157. 1921 May 21. https://collections.library.yale.edu/catalog/32312231.

APA, 6th edition

Barrie, J. M. (James Matthew), 1860-1937. (["Box 6, folder 157"])(1921 May 21)[Duncan, Elsie]. https://collections.library.yale.edu/catalog/32312231.