MLA

Stein, Gertrude, 1874-1946. "Avery Hopwood my idea," holograph. Box 86, folder 1598. n.d.. https://collections.library.yale.edu/catalog/32247978.

APA, 6th edition

Stein, Gertrude, 1874-1946. (["Box 86, folder 1598"])(n.d.)["Avery Hopwood my idea," holograph]. https://collections.library.yale.edu/catalog/32247978.