I have on a number of occasions mentioned Ernst Malmberg (1867-1960), who as early as in the 1910's started auctioning off documents belonging to the archive of the Livonian Governor General. When I visited Uppsala University Library some weeks ago I had the opportunity to looke at volume Y 92, an autograph book which belonged to him. Although it was far from chronological it was possible to get a fairly good idea of how much time he spent in Riga. In April 1909 Malmberg was in Grisslehamn, in August he met with with the author Karl-Erik Forsslund (likely in Sweden), but in late September Malmberg had reached Riga and in September 1910 he was in Helsinki. It would consequently appear that Malmberg spent close to a year in Riga, which certainly would help explain how he could get hold of such a large number of documents.
Y 92 contains the signatures of many celebrities, for example Theodore Roosevelt (whom Malmberg apparently encountered in Chicago in 1911) and the opera singer Christina Nilsson.