MLA

Beinecke, Edwin J. (Edwin John), 1886-1970. Letters from Robert Louis Stevenson to Charles Baxter (In Series I). Box 85, folder 2008. 1883-1884. https://collections.library.yale.edu/catalog/32407459.

APA, 6th edition

Beinecke, Edwin J. (Edwin John), 1886-1970. (["Box 85, folder 2008"])(1883-1884)[Letters from Robert Louis Stevenson to Charles Baxter (In Series I)]. https://collections.library.yale.edu/catalog/32407459.