Proving existing citizenship
First ask the organization requesting proof whether a passport or identity card is sufficient. If a formal determination is needed, collect ancestry records and evidence of past acquisition or loss. The Federal Office of Administration explains forms F/FK for people abroad.
Special rule: treated as German
Section 3(2) StAG also contains a special rule for someone treated as a German citizen by German authorities for twelve years without being responsible for that treatment. This is not something an old document alone can conclusively establish.
MeaNext provides general guidance, not individual legal advice. Only the competent authorities can examine documents, exceptions, security checks and statutory requirements in an individual case and make the official decision.