First papers of surrealism, page 40, Coordinating Council of French Relief Societies, New York, 1942.
View magazine, vol. 5, no. 1, page 32, New York, 1945.
Note the Boîte-en-valise is not an earlier publication. Although the Boîte was published as early as 1941, and Network of Stoppages is included in some inventories of the Boîte, it was only included in editions starting in 1966. (Source: The Complete Works of Marcel Duchamp, Arturo Schwarz, 1997, p. 764)
Also published without copyright notice on the cover of Artforum, December 1963. There is no copyright notice at all, neither on the artwork itself, nor on the magazine as a whole.
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (70 years p.m.a.), Mainland China (50 years p.m.a., not Hong Kong or Macao), Germany (70 years p.m.a.), Mexico (100 years p.m.a.), Switzerland (70 years p.m.a.), and other countries with individual treaties.