Some of Robert's correspondences with Heinrich Schwarz are archived at the Rhode Island School of Design, Museum of Art, Providence, Rhode Island. Other letters are archived with the Bernard M. Bloomfield Papers, 1743-1963 at Winterthur in Delaware. One letter is dated January 7, 1917, and is addressed to Maurice Brix and concerns "the earliest date for the firm of H.I. Pepper and Sons". Other letters between Maurice Brix and Hollis French dated: February 18, 1927; March 4, 1927; and April 4, 1927, inquire about Robert Ensko's publications. The Metropolitan Museum of Art has one piece of silver that was owned by Robert Ensko, Inc.: A two-handled bowl dated between 1700–1710, made by Cornelius Kierstede (1674–c1757). It is MMA 38.63.
