Reynolds 531 tubing
Reynolds 531 has been in production for 70 years (as of 2005, when this article was published), so there's no question that it is true classic! The article suggests that Reynolds developed the process for butted tubing, leading to a separate business group called the Patent Butted Tubing Company. The steel alloy with a high manganese content was introduced in 1932, and the beloved 531 is a further development that reached the market in 1935. Reynolds introduced a heat treated version that was known as 753, allowing the use of thinner tube walls and permitting lighter frames. Further development brought us Reynolds 853, which was a heat treated steel that could tolerate higher brazing and welding temperatures. Reynolds 953 was a stainless tubing, allowing a highly corrosion resistant frame that could also be polished to a chrome-like shine.
for what it's worth... Reynolds is still busy marketing their tubing. I saw their booth at the 2015 NAHBS show.
Steve in Peoria