++++ Social security for migrant workers - Invalidity insurance - Special procedures for applying German legislation - Consideration of qualifications in determining entitlement to benefits - Community provision authorizing the refusal to take account of a qualification obtained in another Member State - Covert discrimination on grounds of nationality - Void ( EEC Treaty, Arts 48 and 51; Council Regulation No 1408/71, as amended, Annex VI, Part C, Point 15 )