As I have mentioned many times before the military historian Hugo Uddgren (1876-1955) in 1919 published the first volume of his Lewenhaupt biography. During the preparations for this volume Uddgren made use not only of the most important archives in Riga, but also went to Saint Petersburg where the archive of the Dukes of Courland was kept at that time. This collection in 1929 came to Riga and is now fond 554 in Latvijas Valsts vestures arhivs. From the perspective of the GNW it is a very relevant archive because the records of the Swedish administration of Courland 1701-1709 have at some point been attached to it. Generally speaking this means that letters to Lewenhaupt as Deputy (and acting) Governor of Courland or as commander of the Courland army can most likely from 1703-1706 (when he was appointed Governor of Riga) be found in LVVA, fond 554. Letters from Lewenhaupt and other officers in the Courland army to the Governor General of Livonia can be found either in LVVA, fond 7349 or in EAA, fond 278. This latter fact can sometimes be really confusing, as Uddgren gives no details about the volumes in which the documents he used were found, only "Swedish archive, Riga" or "Courland archive, Petersburg).
Let me give an illuminating example: On pages 206-210 Uddgren describes the short siege of Selburg (Selpils) by the Wisniowiecki forces in the summer of 1704. His main sources of information for the situation at Selburg before and during the siege are a couple of letters from the garrison commander Carl Lindschöld to Governor General Frölich in Riga. They are dated 7 June, 16 and 28 July. Of these, Uddgren states, the first is found in the "Swedish archive", the second in the "Courland archive" and the third in the "Swedish archive". Considering who they were addressed to it's a peculiar combination, but Frölich may of course have forwarded a copy to Mitau. Anyway, the situation today is as follows:
1. Lindschöld to Frölich, Selburg 7 June 1704 (EAA 278.1.XX-31, p. 156 f.)
2. Lindschöld to Frölich, Selburg 16 July 1704 (LVVA, fond 7349, op. 1, vol. 292, p. 88 f.)
3. Lindschöld to Frölich, Selburg 28 July 1704 (EAA 278.1.XX-31, p. 158 f.)
This is in no way a unique situation and things get even more complicated if one is looking for Selburg letters older than the summer of 1704. Then there is also at least LVVA, fond 7349, op. 1, vol. 315 and the Malmberg papers in Uppsala University library (which contain about a dozen letters from 1701 and 1702).