With respect to there being fewer students who got 'A' grades back in the day, I think that was because there wasn't quite the work ethic (or was that just me?) that there is now... (and certainly not because they were more difficult, they just weren't)
My O levels were in 1984. Every student took maximum of 8 at my school. The only O level I applied myself to was Maths because I found it challenging - and after working through a LETTS do Maths O'Level work book I got a B. For my other subjects I crammed the night before and got B's for those too, except English Language where I got an A. These grades were considered good back then because it wasn' the norm to try quite so hard - this was pre the policy decision to expand further education and higher education.
I don't think it would be possible to get a grade 6 in todays GCSEs by cramming the night before