Entities
- Magnin <San Francisco>
- Letter from: 18.8.1950, San Francisco
- Maison Blanche <London> [Inhaber: Solomon?]
- Letter from: 1.9.1935, Jerusalem
- Marionettentheater <Salzburg>
- Letter from: 16.8.1936, Mondsee (Austria : Lake)
- Mark <San Francisco>
- Letter from: o.D. [1949?], San Francisco (Calif.)
- Markusdom <Venedig>
- Letter from: 3.5.1914, München
- Marquette Building <Detroit>
- Letter from: 24.10.1924, Detroit
- Marx & Spencer <London>
- Letter from: 15.5.1935, Jerusalem
- Matisse Gallery <New York>
- Letter from: 28.4.1947, New York
- Meierei C. Bolle <Berlin>
- Letter from: 22.7.1918, Charlottenburg (Berlin;Germany)
- Mendelsohn & Co. [Mannheimer]
- Letter from: 25.4.1933, Amsterdam
- Metropolitan Museum of Art <New York>
- Letter from: 5.1.1953, New York (N.Y.)
- Metropolitan Museum of Art, The Cloisters <New York>
- Letter from: 28.12.1952, New York (N.Y.)
- Metropolitan Opera <New York>
- Letter from: 22.12.1952, New York (N.Y.)
- Metzgerei Thelen <Libau>
- Letter from: 24.4.1918, Charlottenburg (Berlin;Germany)
- Michel & Cie
- Letter from: 5.8.1915, Charlottenburg
- Michigan University <Ann Arbour>
- Letter from: 16.10.1924, New York
- Michigan University, Architekturfakultät <Ann Arbour>
- Letter from: 29.10.1924, Chicago
- Mills College
- Letter from: 23.4.1942, San Francisco
- Letter from: 2.5.1942, New York (N.Y.)
- Letter from: o.D. [1949?], San Francisco (Calif.)
- Minsterium <Berlin>
- Letter from: 21.6.1920, Allenstein (Germany)
- Missouri Athletic Club <Saint Louis, Missouri>
- Letter from: 21.10.1945, Saint Louis
- Monier-Gesellschaft <Berlin>
- Letter from: 2.9.1918, Charlottenburg (Berlin;Germany)
- Morton House <Grand Rapids>
- Letter from: 1.3.1949, Grand Rapids
- Münchner expressionistische Werkstätten
- Letter from: 2.6.1920, Charlottenburg
- Münchner Kammerspiele <München>
- Letter from: 17.6.1914, Berlin (Germany)
- Museum of Modern Art <New York>
- Letter from: 7.12.1941, New York (N.Y.)
- Letter from: 23.3.1942, New York (N.Y.)
- Letter from: 17.4.1942, New York (N.Y.)
- Letter from: 12.3.1946, Washington
- Letter from: 22.6.1949, New York
- Letter from: 22.6.1949, New York
- Letter from: 16.10.1949, New York
- Letter from: 16.6.1950, San Francisco (Calif.)
- Letter from: 28.12.1952, New York (N.Y.)