MLA

Betjeman, John, 1906-1984. "George V," holograph. Box 3, folder 164. undated. https://collections.library.yale.edu/catalog/17306194.

APA, 6th edition

Betjeman, John, 1906-1984. (["Box 3, folder 164"])(undated)["George V," holograph]. https://collections.library.yale.edu/catalog/17306194.