So is it fair to make an assumption that the end of 1974 may have seen the switch from yellow to black head tubes? All the original early SB frames with a yellow head tube all share the same fork style.
They all have a fully chromed fork with a fully sloping internal fork crown.
The next known frame is SB48 which has had a respray and has a black head tube; but could that black head tube have been yellow originally?
SB58 is probably original and has a black head tube.
Comprehensive records of Raleigh frame numbers is another very useful factor: you can check the age of your Raleigh at the bottom of this page.