I had a quick look at syllabuses for iGCSE(Cambridge) and GCSE(AQA) and the IGCSE seems well beyond the GCSE. Includes topics like integration, set theory and the like. I thought I had seen one that included more probability too but can't see that now. I'm afraid I don't have personal experience of O Levels in the 60s,When I did mine in the late 70s the syllabus looked very similar - although the style of the questions now seem much easier / more accessible than the ones I remember
I looked at an AQA Further Maths GCSE paper and a Cambridge Further Maths iGCSE. The former I am pretty confident I could rattle off very quickly and get them all right (other than silly slips) the second I would have found much more taxing - predominately because it assumes you have memoriised more formulae that I am sure I knew 30 years ago but can't call to mind now.
it seems to me we really need three forms of maths
a) Basic numeracy - the sort of maths everyone needs for day-to-day life. Arithmetic, Mensuration,Fractions, Percentages - this is probably what the Foundation GCSE is aimed at
Maths - probably roughly equivalent to current GCSE - appropriate to most school pupils
c) More Maths (Further/Additional) aimed at people who need it to support their further study.
I see further maths primarily as an enabling course - covering aspects such as Calculus that will be needed for e.g. Maths or Physics A level. I can understand that employers may not value it particularly because you would expect almost anyone who needs it to have a further qualification that supersedes it - but you could see universities for example not accepting someone on a physics degree who didn't have further maths.
I have never really understood the purpose of AS Levels - they have always seemed to me rather like O Levels taken a year later and put in to compensate for the dumbing down of the GCSE (now there is another controversial topic). But on the Maths side I would not say that taking the Further maths syllabus as an AS level would really be a sufficient introduction to A Level maths because it detracts from what you can cover in the A Level syllabus.
Even when I was at school it was the norm to do the Ordinary Maths O level in the 4th Form to allow Further maths to be taken in the 5th Form and that is a pattern that still seems common (I expect my ASD son to take GCSE in Yr 10 at the latest, maybe even Yr 9) You could argue that the Numeracy part of the Maths should routinely be taken around Yr 7 or 8 for able pupils to get it out of the way and allow them to move on earlier to more exciting work. Both my sons (one ASD one NT) find their respective maths very boring because it is so easy!