Race programmes are not in fact always a very accurate reference source!
The actual official size of the Cooper s in this official configuration was 1,275 c.c.
However, most race and rally engine builders over-bored to +20 thou as a matter of course. (n.b. Correction to earlier post, 1293 c.c. is +20 thou, not 60 as I stated earlier: it's been a few years!).
I can try and find out for you as my best friend (and oldest one too, as we have been chums since we were at school in the early 50s!) is one of the leading figures in the official heritage Mini Cooper Register.
As to when, exactly, that the first tuner discovered the slightly extra bore size would have probably been the first day +20 pistons became available!
It is worth remembering that the Cooper s engine was based on the works XSP dry sump. 1,000 c.c. Formula 3 engine, which used forged steel crank and rods, duplex timing chain and the extra stud in the head, which was a special casting to accomodate the larger inlet valves.
From memory, forged pistons were an homologated option for the s, in all three sizes.
In all probability, privateers would have had to wait until Hepolite (e.g.) came up with suitably strengthened +20 thou pistons to allow the extra swept volume; whereas Abingdon would have used them on competition cars from day one.
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