But the post-74 cohort didn't sit the 11+ so how could the comparison be made? Did they guess who would have passed the 11+?
Pre-74 cohort, the top sets in the schools in question had all passed the 11+: that was how it was done.
Post-74 cohort, the top sets emerged by the usual setting mechanisms from comprehensive intake.
The outcomes were the same.
Did a larger % of state kids get into Oxbridge when there were grammar schools?
Depending on the periods you're comparing, yes they did. Parliamentary Briefing Note and shit. There was a massive drop in entry to Oxbridge from state schools starting in 1980. There's some problems with definitions, and you can see from the graph on page two that there's a discontinuity caused by changes to what "state school" included, but the drop from 1980 to about 1988 is well documented elsewhere. That's the cohorts that started secondary school immediately following the widespread abolition of selection in maintained schools, which happened in the early to mid 1970s. The Hartford Scheme and other projects to widen participation from state schools complicate comparisons after that point, too.
Comps could do as well as grammar schools in 1974, but school's are failing the brightest today
Yes. The most interesting metric in the DfE tables for this is to compare "GCSE Equivalent Entries" for high attainers with "GCSE Entries". What you see is the large number of schools boosting their notional GCSE success rate with non-GCSE qualifications, for students who arrive with 5 in KS2 SATs (the definition of "high attainer" in these tables). Such schools tend to have poor value-add in this segment, low EBacc and the students, possibly without knowing it, have large areas of higher education and the professions closed off to them.
My local comp has gone into special measures this week. The report hasn't yet been published, but a look at the data makes one think this is highly likely to be part of the problem (VA below 950 for high-attainers, in a low-FSM, low-ESL, low-Statement school in an affluent suburb).