Worksop closed in 1980. W prefixes continued to be used by Raleigh on Lightweight Unit
bikes built in Nottingham.
Sheldon Brown's poor explanation of the facts(as you cite them)is confusing the SBDU
, created in 1974 and built in 753, 531pro and 531c, with the more mainstream Raleigh operations at Nottingham
for an article on the SBDU at Ilkeston.
I purchased new in 1984 a Lightweight Unit bicycle and and SBDU frameset(531pro)in 1986. Sheldon Brown's frame numbering explanation when compared with my Raleighs - either the Ilkeston or Nottingham built frames is nonsensical.
There are many ex-SBDU and Carlton(Worksop)guys still working in the cycle trade - they are a more reliable source of information than Sheldon Brown appears to be
Most of what I comment upon is from my direct experience: talking to the SBDU people when I went to Ilkeston to order my frame, doing business with the Raleigh Board of Directors in the 1980s and talking to ex-Carlton employees over the last few years when having my Carlton frames(most now sold)restored.
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.