MLA

Stowe, Harriet Beecher, 1811-1896. Letter : Andover, to [James T.] Fields. Box 34, folder GROUP 567, F-2. n.y. Jun 12. https://collections.library.yale.edu/catalog/32308132.

APA, 6th edition

Stowe, Harriet Beecher, 1811-1896. (["Box 34, folder GROUP 567, F-2"])(n.y. Jun 12)[Letter : Andover, to [James T.] Fields]. https://collections.library.yale.edu/catalog/32308132.