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This caligraphed 'M' stands for the initial of Mayerne, which Sir Theodore de Mayerne (1573-1655), compiler of the 'Mayerne Manuscript' MS Sloane 2052, added on folio's, often in the margins in black or red inks. Apparently it refers to text passages that Mayerne seems to have written himself or recipes that he approved of. (Cf. H. Trevor-Roper, Europe’s physician. The various life of Sir Theodore de Mayerne, New Haven & London 2006.)