In 1939 Jacques Seligmann & Co., Inc., was retained by the Clarence H. Mackay Estate's executors to act as agent for the sale of the collection. Mackay was a wealthy American collector of Renaissance and eighteenth-century decorative arts, tapestries, and arms and armor, and he was a trustee of the Metropolitan Museum of Art. A good portion of his collection was originally acquired through Jacques Seligmann & Co., Inc., and Mackay had also been a close friend of the firm's partner and associate, Eugene Glaenzer. As many items had no cataloged provenance, Seligmann enlisted the aid of Stephen V. Grancsay, curator of arms and armor at the Metropolitan Museum of Art, and Carl Otto von Kienbusch to help with the authentication and cataloging of the collection.

This subseries is further divided into nine sections that consist of various categories of correspondence, reports, financial files, inventory and stock notebooks, lists, printed matter, and photographs. Of special note within the Paris office (Jacques Seligmann & Co.) folder in Alphabetical Correspondence (Series 2.4.3) is a reply letter dated August 17, 1939, from the museum director and conservator of the Bavarian National Museum in Munich, Germany. It bears the swastika seal of the Nazi regime and is addressed to César M. de Hauke. The director cordially thanks de Hauke for information sent to him regarding the Clarence H. Mackay Collection but states that the museum is not interested in purchasing any of the artwork. Also of interest in the same folder is a letter, dated March 16, 1939, written by Germain Seligman to Georges E. Seligmann, François-Gerard Seligman, and César M. de Hauke, announcing his recent appointment as adviser for the Mackay Collection and detailing items in the collection and their importance. Germain further suggests they "start right away" contacting Marshal Hermann Goering, who he believes may be interested in acquiring armor for his collection.

The bulk of this series has been scanned with the exception of a General Expenses Ledger.