Thanks for the explanations, airchabum and Trabbi, and especially CF for the super ICAO link. Think I understand it now.
All those years when we thought a Viscount was a VC8, whereas now it is a VISC - makes more sense, just like using the numbers we use every day makes more sense than EA32 or B73A.
Just goes to show
- there are lots of details in aviation
- the details keep changing
- we don't know what we don't know !
Cheers