The answer may be fairly simple. It seems that the place-name was only first noted in the 1970s, featuring in the well-known translation of the RFA by Bernhard Walter Scholz and Barbara Rogers, which, in turn, was based on Kurze's edition from 1895. 3/5

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