Reynolds is the name of the tubing company that sold tubing to the bike company. 531 is the brand name of the allow used. It's similar to standard 4130 chrome moly, but it's stronger, being manganese molybdenum (sorry, not chrome vanadium!). "R" designates that the tubes were "standard diameter," but with thinner walls than standard. The thinner walls mean a lighter frame, but more importantly more flexible, tube for tube. It's a very desirable frame because of those tubes. The only think of the vintage from Reynolds was 753, which was the same dimensions but a stronger steel allow. Not stiffer, but stronger; able to take deeper flexing without damage.