Here (Beckenham, Kent) A, B, and C lists appeared in rolls in force 1948-9, 1952-3, 1955-6, and 1977-8, with a slew of supplementary pages in 1996-7.
From 1952, a "Y" indicated young electors, who could only vote after 1st October. From 1970, the actual date from which they were eligible was given, useful if you only have the year.