MLA

Lindbergh, Anne Morrow, 1906-2001. Lowell, Robert. Box 12, folder 243. 1948. https://collections.library.yale.edu/catalog/10016097.

APA, 6th edition

Lindbergh, Anne Morrow, 1906-2001. (["Box 12, folder 243"])(1948)[Lowell, Robert]. https://collections.library.yale.edu/catalog/10016097.