Written documentation of ALCLAD aluminum markings in 1941 is provided in Aircraft Sheet Metal Work: Bench and Repair Work, H. Edward Boggess, New York, Pitman Publ. Corp., 1941. (http://hdl.handle.net/2027/mdp.39015018277460).
From page 13 included below: “The letters AL or ALC preceding the alloy designation indicate that the material is Alclad, which means that it has been surfaced on both sides with pure aluminum.” And further on the following page, “Thus a piece of metal labeled AL-17ST-ANA would be heat-treated, Alclad dural which conforms to the Army and Navy specifications and comes from the mill at Alcoa, Tenn.”
Numerous photographs show the “ANA” mill/plant marking, but this photograph is the only example discovered to date showing the “ANK” mill/plant marking. (Source: National Archives, http://research.archives.gov/description/535581)