I think the Kauga mentioned in the extract S_L linked to is a red herring. The entry doesn't mention Germany, only "Bornou" (which I haven't located), but crucially, the latitude and longitude given place Kauga close to the border between Chad and Sudan, near the southern edge of the Sahara Desert.

I'm also not convinced by the Hansa suggestion. The Hansa (Hanseatic League) existed to foster trade, rather than as a political or geographical entity, and although individual cities were (and are) proud of that heritage, I'm not sure that there was ever a region of Germany known as "Hansa". In any case, the initial letter isn't like the one in Harry.
Having expressed doubts about two suggestions, I'm afraid I can't do the decent thing and come up with one of my own. But I wonder if Rebecca's maiden name might be helpful - obtainable from a child's birth certificate, and of course her marriage certificate, if there is one.
Arthur