MLA

Conrad, Joseph, 1857-1924. Thys, Albert. Box 33. 1889-1890. https://collections.library.yale.edu/catalog/2014002.

APA, 6th edition

Conrad, Joseph, 1857-1924. (["Box 33"])(1889-1890)[Thys, Albert]. https://collections.library.yale.edu/catalog/2014002.