It's a personal bit of deductive work. I ran across a '77 Grand Prix which I am rebuilding into a city bike/grocery getter. When I looked up the serial # WL6086 I found that W means worksop plant, L the fortnight it was built (Jul or Aug) and 6 being the year of manufacture. Now according to legend (and Sheldon Brown) the Worksop plant was dedicated to building only the high end frames with Reynolds 531 tubing. The Super Course was the only mixte frame Raleigh built with 531 tubing, and discontinued it after 1976. That plus the fact that my bike has no kickstand plate while the Grand Prix was always spec'd with one leads me to the suspicion that it was a leftover Super Course frame they decided to sell off the following year. I don't know if that's true, but nobody has been able to provide a better explanation for the serial #. At any rate it's fun the think I might have benefited in some small way from a corporate misjudgment. It also feels a lot like the old 531 Follis I rode during my college days.