I see from the transfers that yours is a 531 SL frame. I can't remember when Reynolds started making the SL version of 531 but it might not have been as early as 1974.
Mark, you're hunch is correct -
531"Special Lightweight"or SL was introduced in 1978.
"Designer Select"was a builder's choice
mixed tubeset of 531C and 531ST - introduced in 1981.
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.