I did write
differing exams applied in Scotland)….
but yes, I ommitted Northern Ireland, Channel Islands and other exceptions
O-level grades were originally ABCDE with FGH as fails but considered too fine a distinction at O level - hence ACE in my area .... differing regional examining boards varied and some used numbers, I think.
AO were for Lower-Sixth - in between O and A and a means of showing progress after 1 year.
'In my day' the exams tested part of the syllabus - all of which was expected to be taught - but not all parts could be in the paper each year: 'Past Papers' were published for practice use (then photocopied!). Now the boards seem to tell the teachers the questions and answers, according to press coverage - and only those parts are then taught. - saves time for all the form filling that politicians now require.
'Nuffield Science (from another educational foundation) encouraged thinking - and their text books asked questions but omitted answers - no longer a reference book (at least in Physics, which I taught) It included exam questions like " You were taught about [a type of telescope] ... this is a different type - explain how it works: think how parent's would complain about that today! (the pupils complained then)
We did Food Science in Nuffield A-level Chemistry (when I was a pupil) - and brought in fresh apples for each lesson ... it broke their budget but we ate a lot of apples! It wasn't done that way then next year 8-(
I left school teaching about 30 years ago (it probably shows) and went into industrial (broadcast) product training: no marking! but constant assessment of the teacher.