Wikipedia
The GCE S-level, Scholarship level, or Special paper was a British public examination taken by the most able A-level students. The S-level was typically used to support UK university entrance applications, though in practice it was directed almost exclusively to Oxford or Cambridge applications. Results were graded 'Distinction' (1), 'Merit' (2) or 'Unclassified' (U).
Up until and including 1960, the actual mark in steps of 5 was provided to candidates. The S level Higher Maths papers were not marked unless at least 75(Distinction) was secured in the A level Pure and/or Applied mathematics papers. The marks were normalised, but usually completion of 2 or 3 questions of the 10 was stated by the examiners on the paper to be sufficient to secure a distinction (75). The highest possible score was 90 for ten excellent answers, due to normalisation. The subject matter was identical to the A level syllabus, but the questions very considerably harder.
Up to 1962, the main objective of the S levels was to permit the Ministry of Education to allocate 400 State Scholarships for the best performance of all those examined at A (and of course S) level in that specific year.
Although it was a separate paper, marked and graded in isolation from the A-level, it was not commonly a standalone qualification and was usually attempted only by candidates who were also sitting an A-level in the same subject at the same time and who were likely to obtain an "A" grade (the top grade) in that examination (and results only given to candidates who had actually achieved an "A" or a "B"). Given the very small, selective entry for the papers, the S-level was only offered in a small number of mainstream subjects.