• Note this is a larger copy than the photo in the ad. Larger photo has a photographer's mark at lower left--looks like Reade, NY.
• RCA didn't mark the ad as copyrighted; Billboard copyrights would not apply to the ad.
• There are no copyright marks on the ad.
• US Copyright Office page 3-magazines are collective works (PDF)

"A notice for the collective work will not serve as the notice for advertisements inserted on behalf of persons other than the copyright owner of the collective work. These advertisements should each bear a separate notice in the name of the copyright owner of the advertisement."

This advertisement (or image from an advertisement) is in the public domain because it was published in a collective work (such as a periodical issue) in the United States between 1929 and 1977 and without a copyright notice specific to the advertisement. Unless its author has been dead for several years, it is copyrighted in jurisdictions that do not apply the rule of the shorter term for US works, such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties. See this page for further explanation.