Fascinating stuff, and we could easily get into memory lane on all this.
It is best to concentrate on the main point, on which players in years past could reasonably be expected to have reached IM level, were it not for norm chances had been more restricted before the 1980s.
In 1971 and 1981, all the players who reached gradings of 225 achieved their IM title, and in most cases GM title. This is to be expected. After all, to get an IM title, a player will need a rating of 2400, the equivalent of a 2400 FIDE rating. In the 1968 rating, what is striking is that Peter Lee and Adrian Hollis went beyond 225, while Michael Franklin was just a grading point below. We can say that these players were basically at IM strength. In the sixties though, they would only have had limited opportunities to play international tournaments at IM level. Certainly in the 1981 cohort, most of the players who reached 220 achieved IM level.
We come now to players in these three years who achieved gradings between 220 and 224. Perhaps on these particular years, they are not quite at the strength of achieving IM titles, but it is reasonable to suppose that if these players could put in some hard work to achieve IM strength (these days measured by norms and the FIDE system), they would probably achieve IM status.
On the three lists given, we have:
1968 - Clarke
1971 - none!
1981 - G Lee, J Littlewood, J Nicholson, D Rumens, K Harman
It is quite reasonable to suppose that if they had repeated this performance, earlier or later, then, given time and opportunities, they would have reached IM level.
We note though the death by drowning of Ian Wells, then aged 17, who had a grading of 220. He would very definitely have become a GM, had he lived.
What is striking in the 1971 list was that there were a few established GM and IM players, highly experienced in international chess, and a large number of players graded between 215 and 219. The younger players generally achieved IM status. The older players generally did not. These more established players did not lack talent. More likely, they had little incentive to push that little bit harder against international level opposition. These days, it is easy enough to play grandmaster opposition, even in team events. I have recently played against GMs in the London League and in the Middlesex League, in the last couple of months, for example, and I am hoping very much that my team will get promoted in the 4NCL. There were just not the opportunities for all this in the 1960s and early 1970s. Many strong players were not pushed hard enough.
Anyway, statistical challenge.
Players who reached 225 strength, but did not achieve IM status? Players who reached 220 twice, but did not achieve IM status? And, before the modern BCF/ECF grading system, the equivalents of these levels of strength?