English: An assortment of Alnicomagnets produced by The Arnold Engineering Co. in 1956. These are probably Alnico 5 magnets, and show the large range of shapes manufactured for use in devices such as magnetron tubes and small DC electric motors. Invented during World War 2, Alnico 5 was at that time the most powerful magnet in the world. It's development created a new generation of compact permanent magnet motors and loudspeakers, devices which before the war had used DC electromagnets. Alterations to image: cropped out surrounding ad copy.
This image is from an advertisement for The Arnold Engineering Co. without a copyright notice published in a 1956 magazine. In the United States, advertisements published in collective works (magazines and newspapers) are not covered by the copyright notice for the entire collective work. (See U.S. Copyright Office Circular 3, "Copyright Notice", page 3, "Contributions to Collective Works".) Since the advertisement was published before 1978 without a copyright notice, it falls into the public domain.
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 (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.