I saw the reynolds site, I assume 501 is an older method and the 531 is newer (although they talk about 631 now on their site)
I was looking for information for a bike I saw on ebay then I read the term on a post on this site:
I lusted after the Raleigh Record Sprint - all anodised black and gold. Even though my bike was way better, and it was really a mid range 501 bike, it just looked so cool.
http://www.bikeforums.net/showthread...+Record+Sprint
Then I realised I had no idea what a "mid range 501 bike" meant, I assumed by the way it was used in the post, that it was a commonly used term. I guess not.
Cheers, Steve