yep, the day/night scheme was changed between end of Dunkirk and beginning of the BoB, sometime in early June.
for the record The Spitfire Story, by Alfred Price lists the diff between 87 octane fuel and 100 as
"giving no improvement at or above 16,500 foot full throttle height of the merlin II and III engines fitted to the spitfires, below that altitude the maximum boost pressure could be raised from 6 1/4 pounds to 12 pounds without causing detonation in the cylinders; the resultant extra power increased the maximum speed by a useful 25mph at sea level and 34mph at 10,000 feet"
and being supplied to frontline squadrons from spring 1940.
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