531 Professional.
531 Pro or"P"was produced from 1981 to 1986 - tubesets could quite easily have been on a builders shelf for some time after.
A well known framebuilder (a friend he's retired now) has at least two sets in his garage.
531 Pro was the remarketed version of the 531 SL (Super Lightweight) tubeset. 653 did replace 531 Pro - in 1986. 653 however was not really a tubeset as such: bit of a hybrid I recall - 531 maintubes and fork with a 753 rear triangle.
My 1986 SBDU is 531 Pro: nearly as light as 753.
Judging by the headbadge (if original), I'd place the frame earlier rather than later.
Roadking.
