The 40 has to do with years of same old and not head length.....I am currently thinking of joining in with Fred A and marketing some 40+ scotch to go with the line
Below is from from Airflo;
Recent fly rod design and materials have revolutionised fly rod performance and changed the way we cast.
Yet as a line manufacturer, the line rating system that we must adhere to was created over 40 years ago when many people still used rods built from cane or fiberglass.
Airflo realized that to maximize your true potential with modern fly rods, the flyline design had to be radically upgraded - welcome to the new Forty Plus line.
Ask almost any flyfisher what cast they would like help with and they will tell you it is the distance
cast. By taking the advantages of a shooting head and blending them into an advanced weight forward design, we have created a distance casting flyline for everyone.