OK, so bear with me - In Kurze's edition, the footnote referring to the missing location is labelled 'u'. Further down the same page, however, another footnote has that exact same letter, and refers to one of the delegates at the meeting: a certain Osfrid, 'son of Heiligen'. 4/5

So the 'Treaty of Heiligen' may be the result of a modern mix-up, which has since begun to lead a life of its own. As such, would it be better to just refer to the event as the 'Treaty of the Eider' instead?