MLA

Balʹmont, Konstantin Dmitrievich, 1867-1942. [Postcard: 1923 December 1, Paris: Konstantin Dmitrievich Balʹmont to D[agmar] Chahovskoy, Sèvres]. Box 3. 1923 December 1. https://collections.library.yale.edu/catalog/10082166.

APA, 6th edition

Balʹmont, Konstantin Dmitrievich, 1867-1942. (["Box 3"])(1923 December 1)[[Postcard: 1923 December 1, Paris: Konstantin Dmitrievich Balʹmont to D[agmar] Chahovskoy, Sèvres]]. https://collections.library.yale.edu/catalog/10082166.