the 531 sticker is an old sticker - pre 531ST or 531Comp, so that would suggest (if the sticker was pukka) a frame of 25-30 yrs old +. But the paintwork looks to be too new for that.
The internal brake cable guides suggest a modification post mid 80's when this became fashionable and was generally dropped early 90's when fashion and the fact that these guides (sleeves) filled with water and caused sticking rear brakes.
The seat lug metal looks quite chunky. i would expect a master builder to have used finer (thinner) lugwork.
531 frames generally use 27.2mm seatpins for butted (27.0 on some) and 26.4 for plain gauge 531. Good columbus also uses 27.2 and cheap columbus (aelle/zeta) used 26.4. Some clunkers which were actually finished quite well, but used heavier tubing (eg. Falk) also used 26.4. If youve got a 27.2 seatpin, then I'd say 99% youve got a good un.
Weight is not the be-all-and-end-all. The 80's concentrated on style and durability, hence some top bikes weighed in at 23lbs. Columbus SPX was a quality, heavy bombproof tubing used on larger, expensive frames.
All the above is not definitive - its based on my knowledge/experience. others may say different and either might be correct.
Frames been powder coated so the 531 sticker is a repro - To me it looks like something from Dawes or Claud Butler from around the late '80's. I've got stuff with exactly the same lugs, rear stay finish and so-on.
Looks like a Moore & Large era frame