Even during the 1950s, at least one cereal manufacturer was adding sugar to raisins:
Judging by the lack of graphic art on the box, a different commercial for the same brand (shown below) may have been produced earlier. It doesn’t mention sugar on the raisins.
The U.S. English vernacular may have been different from the way it is now, too. Seriously, someone who did creative work at an ad agency believed the American Public would buy cereal endorsed by Jake the Flake, and the agency’s client approved it. If this commercial were produced now, we’d call the name of the male narrator a collective brain fart: