The State Treaty for the Re-establishment of an Independent and Democratic Austria (Vienna, 15 May 1955) contains the following articles:
Part I: - Political and Territorial Clauses
Recognition by Germany of Austrian Independence
The Allied and Associated Powers will incorporate in the German Peace Treaty provisions for securing from Germany the recognition od Austria's sovereignty and independence and the renunciation by Germany of all territorial and political claims in respect of Austria and Austrian territory.
Prohibition of Anschluss
The Allied and Associated Powers declare that political and economic union between Austria and Germany is prohibited. Austria gully recognizes its responsibilities in this matter and shall not enter into political or economic union with Germany in any form whatsoever.
In order to prevent such union Austria shall not conclude any agreement with Germany, nor do any act, nor take any measures likely, directly or indirectly, to promote political or economic union with Germany, or to impair its territorial integrity or political or economic independence. Austria further undertakes to prevent within its territory any act likely, directly or indirectly, to promote such union and shall prevent the existence, resurgence and activities of any organization having as their aim political or economic union with Germany, and pan-German propaganda in favour of union with Germany.
Frontiers of Austria
The frontiers of Austria shall be those existing on 1st January, 1938.
When Austria was considering, and being considered for, EU entry in 1995 this treaty formed a basis of argument against her entry. However this argument was overcome because of the four enforcing powers of the treaty, to wit France, United Kingdom, United States and the Soviet Union, the three extant powers withdrew their treaty objections. The fourth enforcing power, the Soviet Union, no longer existed and thus no longer could provide objections. A legal quibble perhaps, as Russia still held the former Soviet permanent seat on the U.N. Security Council, but a successful one.
A referendum by the Austrian people then voted in favour of entry, and the corresponding diminishment of their permanent neutral status, by about 66%.
Article 37 deals with which additional nations can accede to the Treaty and become an additional Associated Power in regards to the Treaty, but this warrant is limited to countries which were a United Nation on May 8, 1945, by virtue of being at was with Germany. As best I can determine this warrant has not been exercised by any other power.