MLA

Ernest Augustus, King of Hanover, 1771-1851. Autograph letter, signed, to Princess Augusta. Box 30, folder 139. no year June 13. https://collections.library.yale.edu/catalog/10000204.

APA, 6th edition

Ernest Augustus, King of Hanover, 1771-1851. (["Box 30, folder 139"])(no year June 13)[Autograph letter, signed, to Princess Augusta]. https://collections.library.yale.edu/catalog/10000204.