The registry office 1 in Berlin requests proof (document) of how you aquired German citizenship, aka naturalization or birth. A notarized (certified) copy can scanned and sent (I suppose, electronically) to the contact person.

You may want to find out what type of notarization they are accepting. For citizens residing in foreign countries, they often require it to be notarized by the German consulate responsible for your district.

It doesn't say in the request, but as we all know, Germans are very picky on details.

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