MLA

Barrie, J. M. (James Matthew), 1860-1937. Clemens, Samuel Langhorne: calling card, signed. Box 16, folder 575. undated. https://collections.library.yale.edu/catalog/2005538.

APA, 6th edition

Barrie, J. M. (James Matthew), 1860-1937. (["Box 16, folder 575"])(undated)[Clemens, Samuel Langhorne: calling card, signed]. https://collections.library.yale.edu/catalog/2005538.