From Wikipedia: The Royal Commission on Historical Manuscripts (widely known as the Historical Manuscripts Commission, and abbreviated as the HMC to distinguish it from the Royal Commission on the Historical Monuments of England), was a United Kingdom Royal Commission established in 1869 to survey and report on privately owned and privately held archival records of general historial interest. Its brief was "to make inquiry as to the places in which such Manuscripts and Papers were deposited", and to report on their contents. It remained in existence until 2003, when it merged with the Public Record Office to form The National Archives. Although it technically survives as a legal entity, its work is now entirely subsumed into that of The National Archives.
Now part of The National Archives
More information on the Wikipedia page