The serial number is on the bottom bracket.
An Ilkeston built version will start with SB followed by a number, the numbering was sequential from 1 up to about 8300.
If you find one with what seems like two random letters followed by 3 numbers that is a frame built for a sponsored rider eg JR 178 would be Jan Raas first bike for the 1978 season.
If it starts with a W the frame was built in Worksop, borrowed details below from elsewhere on site.
I have to say, and I've said it so many times on RetroBike
, I have real problems with this document.
It does not correlate to any
of my Raleighs, SBDU frames or LU bikes, I have had them all from new (SBDU ordered in person by me at Ilkeston).
Good luck with the build all the same.
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, 1975 Bill Philbrook tourer, 2 x very early Roberts (mid/late 1960s), 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.