The Treaty

In Article 26 of the Multilateral Austrian State Treaty of May 15, 1955, Austria promised:

In so far as such action has not already been taken, Austria undertakes that, in all cases where property, legal rights or interests in Austria have since 13th March, 1938, been subject to forced transfer or measures of sequestration, confiscation or control on account of the racial origin or religion of the owner, the said property shall be returned and the said legal rights and interests shall be restored together with their accessories.

Austria has failed to live up to its treaty obligations. It is incumbent upon the United States to assert its rights under the 1955 treaty and insist that Austria provide due process to the victims of Nazi persecution, especially those like Maria Altmann who have been loyal U.S. residents and citizens since they fled from Austria over 60 years ago. 

In his May 15, 1959 letter regarding the settlement of Article 26 claims for restitution, U.S. Ambassador to Austria H. Freeman Matthews concluded:

My Government has instructed me to advise you that it may approach the Austrian Federal Government in the future in connection with the settlement of individual claims asserted under Article 26 of the State Treaty which are not presently known to my Government and do not fall within the classes and categories of claims enumerated in paragraphs 1 and 2 of Section A of your note.

In other words, the U.S. reserved the right to assert unknown claims, such as the ones for the Bloch-Bauer's paintings. The fact that the Austrian government had lied to the heirs and had falsified the provenance of the paintings was not revealed until last year, so these claims fall within the category of claims "not presently known" in 1959.