Alternative class names...

Yes, the 505's predecessor was the Coronet, an 18 footer that was designed as something a bit like a stretched 14 (the back end is like a Westell 14, with a longer bow). His own writing doesn't mention any N18 involvement. Instead, he arrived at  the 18' length because it was a compromise between the long FD-type Euro boats and the UK style of boat.

The Caneton class was a development class, 5.05m long. Most of them were hard chined and in the IYRU trials they did quite well, but were not up to the standard of the newer boats. The Caneton class was also concerned that development classes were becoming too expensive. They therefore asked Westell to cut the Coronet down into a one-design boat (changing the shape of the flare as they did so) that would fit the Caneton class rules, and called it the Caneton Rapide for a little while.

Most of this information comes from Westell's own writing about the boat, and from contemporary mags.