As much information as you can shake a stick at
Reynolds 531 tubing was at "peak" popularity in the era from around 1955 to 1975. During that era, the bike that won the Tour de France would have Reynolds 531 tubes about nine out of ten years.
Around 1974, Reynolds was making about nine versions of 531, including three "straight gauge" sets. There was a straight gauge set using 0.91 tubes, one with 0.81 tubes and a light set with 0.71 tubes.
One of the lightest tube sets of that era were Columbus Rekord straight gauge tubes with a thickness of just 0.50. That resulted in a frame and fork weight of just 3 1/2 pounds (plus the weight of the lugs). That is two pounds LESS than a popular version of Reynolds 531 double butted tubing, which was 0.71 thick, and 1.02 thick at the butts. Removing the "thick" area at the end of each tube reduced tube weight.
There is a myth that the quality of steel tubes is related to the thickness of the walls, so that a set with 0.91 walls is "inferior" to a set with 0.71 walls. Not true. Reynolds made only ONE quality of Reynolds 531. The purpose of offering a wide range of tube thicknesses and weights was to permit bike designers to use the tubes that best meet the needs of the rider. A bike designed for "loaded" touring, a bike designed for the "Tour de France", and a bike designed for use on an indoor track record attempt have different needs. And, sometimes, a heavier tube is the BEST tube for a given purpose.
A complete Reynolds tubeset included the top tube, seat tube, down tube, chainstays, seat stays, two fork blades, a head tube, and a steerer (but NOT the lugs). The lightest complete set in 1974 was the 531 SL with a weight of 3.75 pounds. The heaviest double butted set weighed 5.37 pounds. The heaviest "straight gauge" set would likely come in at about six pounds.
Of course, the actual "finished" weight of the frame depends on its size, and the lugs used. A size 60 bike using Reynolds 531 could weigh almost a pound than a size 52 bike.
On that note, I'd grab a 531 frame, build yourself a decent bike and forget about weight.