MLA

Beinecke, Edwin J. (Edwin John), 1886-1970. Robert Louis Stevenson's flageolet (In Series III). Box 127, folder 2112. undated. https://collections.library.yale.edu/catalog/32470159.

APA, 6th edition

Beinecke, Edwin J. (Edwin John), 1886-1970. (["Box 127, folder 2112"])(undated)[Robert Louis Stevenson's flageolet (In Series III)]. https://collections.library.yale.edu/catalog/32470159.