MLA

Cortissoz, Royal, 1869-1948. Gilbert, Cass. Box 6, folder 223. 1924-30. https://collections.library.yale.edu/catalog/17197463.

APA, 6th edition

Cortissoz, Royal, 1869-1948. (["Box 6, folder 223"])(1924-30)[Gilbert, Cass]. https://collections.library.yale.edu/catalog/17197463.