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.
Co-founder of a V-CC section, member South Eastern Road Club and owner of - 2005 Bianchi Reparto Corse (modern), 1985 Raleigh SBDU Pro Super, 1984 Raleigh LU Corsa, 1980 Allin Stan Butler Special, 1978 Ron Cooper, 2 x very early Roberts (late 60s/early 70s), 1975 Bill Philbrook tourer, 1966 Raleigh Superbe Roadster, 1964 Allin Stan Butler Special Belgique, 1951 Hobbs of Barbican S/C, 1950 Hobbs of Barbican S/C, 1947 Hetchins Super Special, 1908 Centaur Featherweight. 1988 Specialized Stumpjumper.