Reynolds Steel
this answer needs completion its just notes at this point
Reynolds butted tubing was first patented in 1897.
There are many different grades of Reynolds steel. The most commonly known is 531 (pronounced "five, three, one") which was first produced in the 1935 but is no longer available outside of New Old Stock, or by Special Order. This steel was also used on Jaguar XKE car chassis, and assisted in 27 Tour de France victories. The replacements are 520 and 525, which are similar to 531 but can also be welded.
list some other codes and their meanings and usages 753 (required certification by Reynolds), 953, 725, 631, 853, 525.
explain the numbers 531 gets its name from the composition. Five parts manganese, three parts carbon, and one part molybdenum.
Advantages
- Classy steel tubing
- Considered as a "forgiving" material for building and daily use
Disadvantages
- Many different grades available
- Heat treating requirements vary across different grades
- Some grades do not permit cold-setting of frames
- Steel can suffer from corrosion issues
References
http://bikeretrogrouch.blogspot.co.nz/2013/12/reynolds-tubing.html
https://en.wikipedia.org/wiki/Reynolds_Cycle_Technology#Tubing_types Big list of tubing codes here.