Nyanza. See here. Again staying with Sandra Miesel's dates for convenience, "The Game of Glory," set in 3035, tells us that:
"...Nyanza...had been colonized some 500 years back during the breakup of the Commonwealth. It had been made an Imperial client about a century ago, a few abortive revolts were crushed, now there was only a resident - which meant a trouble-free but unimportant and little visited world."
-Poul Anderson, Captain Flandry: Defender Of The Terran Empire (New York, 2010), p. 306.
This means that Nyanza was colonized in the same period as Avalon and, indeed, Sandra Miesel's Chronology of Technic Civilization shows us:
26th Century "Wingless"
"Rescue on Avalon"
Colonization of Nyanza (p. 401)
One earlier main period of colonization was immediately after the discovery of hyperdrive in the twenty second century. That period is called "...the Breakup..." in the Chronology (p. 399) but is this term used in the texts?
Cairncross cannot have been involved in crushing any revolt over a century ago so it seems as if Nyanza revolted again soon after Flandry had been there? I will continue to check the text of "The Game of Glory."