Noble I'd guess that the further maths grew out of what was called by my teachers "Paper 5" which (I think) was a pass/fail thing?
When I did GCSE my board had 4 exams 1,2,3,4.
Lower tier did 1&2
Intermediate did 2&3
Higher did 3&4
At my school most of the top set did the maths GCSE in November. Those who didn't in the top set (which was those not on for an easy A) would do it in the summer with the mysterious paper 5. I think you couldn't take it without doing the whole GCSE and then if you passed you got a little 5 next to the A. If you failed you would just get the GCSE grade.
I don't know how easy it was to pass.