MLA

McKay, Claude, 1890-1948. Spain. Box 16, folder 498. n.d.. https://collections.library.yale.edu/catalog/16742374.

APA, 6th edition

McKay, Claude, 1890-1948. (["Box 16, folder 498"])(n.d.)[Spain]. https://collections.library.yale.edu/catalog/16742374.