MLA

Hearn, Lafcadio, 1850-1904. Letter : Kumamoto, Kyushu, Japan, to Roberts Brothers, Publishers, Boston, Mass.. Box 18, folder GROUP 368, F-2. [18]92 May 23. https://collections.library.yale.edu/catalog/17234166.

APA, 6th edition

Hearn, Lafcadio, 1850-1904. (["Box 18, folder GROUP 368, F-2"])([18]92 May 23)[Letter : Kumamoto, Kyushu, Japan, to Roberts Brothers, Publishers, Boston, Mass.]. https://collections.library.yale.edu/catalog/17234166.