MLA

Wescott, Glenway, 1901-1987. Poulenc, Francis. Box 92, folder 1377. n.d.. https://collections.library.yale.edu/catalog/17065018.

APA, 6th edition

Wescott, Glenway, 1901-1987. (["Box 92, folder 1377"])(n.d.)[Poulenc, Francis]. https://collections.library.yale.edu/catalog/17065018.