I've searched the British Newspaper Archive and this is the earliest mention I can find of Steve (or Stevie) Winwood. It comes from the Birmingham Mail for 3 December 1963.
The books say the Spencer Davis Group was originally called the Rhythm and Blues Quartet, but it seems that in practice two names were given prominence even above the name of the band.
Where space allows in small ads, the band's billing was:
Spencer Davies. Steve Winwood.But here, where they had to get a second act in, just names were used. And at 15 years of age Steve Winwood already had his name above the name of the group too.
R & B Quartet