I don't know if that's just "555" or "555T" but my Raleigh tourer is 555T which is just a butted cro-mo tubing made my reynolds. It's pretty light for a touring bike. I guess it would be a more economical option then a bike built for touring out of 531? Not sure but that's what I have to offer!
That does say "design and engineered by Raleigh" so maybe it's a copy of the Reynolds? Sorry I don't have more to say.