MLA

Bryher, 1894-1983. Beach, Sylvia. Box 102, folder 3728-3729. 1931-60, n.d.. https://collections.library.yale.edu/catalog/2013609.

APA, 6th edition

Bryher, 1894-1983. (["Box 102, folder 3728-3729"])(1931-60, n.d.)[Beach, Sylvia]. https://collections.library.yale.edu/catalog/2013609.