When using emergency boost pressures in excess of 6.25 lbs/sq.in. 100 octane was required, therefore if a pilot recorded use of emergency power it necessarily follows that the aircraft was fueled with 100 octane fuel.
Operational Notes for Pilots on Merlin II and III, January 1939 (thanks 41Sqn_Banks)
Pilot's Notes, Merlin II, III and IV, 4th Edition, April 1940, page 6.
Spitfires were cleared for use of 100 octane fuel for improved take-off in September 1938 just a month after the introduction of the type into service.
By 12 December 1939 100 octane fuel was approved for Spitfires, Hurricanes and Defiants.
100 Octane Fuel - Issue of.