Sargasso of Lost Starships," "The Star Plunderer" and two early Dominic Flandry stories (see here and here). In 2011, "Sargasso..." was collected, for the very first time, in Baen Books' The Technic Civilization Saga, because, like "The Star Plunderer" and the Flandry series, this story counts as an installment of Anderson's History of Technic Civilization whereas "Lord of a Thousand Suns" does not.
However, "Lord..." and "Sargasso..." have certain features in common: an Imperial Solar Navy and a visit to an ancient, mysterious, powerful planet. To the original readers of Planet Stories, there was not as yet any Technic Civilization future history but there were several space operas set within an interstellar Empire and "Lord..." might have been perceived as just another of those.
For a full listing of Poul Anderson's Planet Stories stories, see here.