Note: every manifestation of a letter is counted, so the number of documents in a repository may be larger than the number of letters.

Repository Number of documents Further details
Magyar Országos Levéltár 3 City: Budapest
Country: Hungary
Maine Historical Society 1 Alternative names for repository: Brown Library
City: Portland, ME
Country: United States of America
Marsh's Library 24 City: Dublin
Country: Republic of Ireland
Massachusetts Historical Society 36 City: Boston, MA
Country: United States of America
Médiathèque de Sedan 1 City: Sedan
Country: France
Médiathèque du Grand Troyes 5 Alternative names for repository: Mediatheque du Grand Troyes Les bibliothèques de l'agglomération troyenne
City: Troyes
Country: France
Moravian Archives - City: Bethlehem, Pennsylvania
Country: United States of America
Moravská zemská knihovna v Brně 1 Alternative names for repository: Moravian Library
City: Brno
Alternative names for city: Brünn
Country: Czech Republic
Musée des Beaux-Arts de Caen 3 City: Caen
Country: France
Musée royal de Mariemont 2 City: Morlanwelz
Country: Belgium
Museum of the History of Science 24 Alternative names for repository: MHS Museum of History of Science
City: Oxford
Country: England, United Kingdom
Museum Plantin-Moretus 8 City: Antwerp
Country: Belgium
Museum voor Communicatie 83 Alternative names for repository: Museum of Communication Postal Museum
City: The Hague
Alternative names for city: Den Haag
Country: Netherlands
Muzeum Komenského v Přerově 2 Alternative names for repository: Comenius Museum in Přerov
City: Přerov
Country: Czech Republic
Muzeum Narodowe, Biblioteka Czartoryskich 2 Alternative names for repository: Czartoryski Library
City: Kraków
Alternative names for city: Cracow Krakow
Country: Poland