The advertisements are most likely in the public domain. It's true that the 1976 Copyright Act expressly requires that advertisements in magazines have separate copyright notices. But your advertisements (1918-1962) are subject to the 1909 Copyright Act, which had no rules for magazine ads. (See Sec. 3 of the 1909 Act.)
How do they become public domain? First, the ads published from 1918 through 1927 are automatically in the public domain as the copyright has expired. Second, there is caselaw under the 1909 Act that holds that a periodical does not acquire copyright in its ads, even if the magazine's art department created the ads. Third, even if the ads published between 1928 through 1962 were protected by copyright, they would be in the public domain if the copyright was not renewed after 28 years. Only a small percentage -- between 7 and 15% -- of copyrighted works were renewed, so the odds are in your favor. P.S. You can hire the Copyright Office to search the renewal records.