Yesterday’s Mystery Persons were the Nicholas Brothers.
From Wikipedia: The Nicholas Brothers were an entertainment act composed of brothers, Fayard (1914–2006) and Harold (1921–2000), who excelled in a variety of dance techniques, primarily between the 1930s and 1950s. Best known for their unique interpretation of a highly acrobatic technique known as "flash dancing", they were also considered by many to be the greatest tap dancers
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