16 lbs was cleared in Spit VB in late 1942. The question is about +12 lbs. I think actually there would be hard to find any evidence if +12 lbs was allowed from begining of service SPitfire Mark VB or it was allowed somewhere later? I suppose that +12 lbs was emergency boost in Merlin 45 from begining of service - so from 1941 year.
In previous Spitfire types - +12 lbs emergency was introduced with used 100 octan fuel in Merlins. So all Spits MK I which were allowed to used 100 octan fuel got new power settings - + 9lbs for nominal power and +12 lbs for emergency. The same Spitfire MK II which had +12 lbs emergency boost since begining. I think with Spitfire MK VB was the same - +12 lbs was emergency boost since begining, then incrased for +16 lbs in late 1942 ( and +18 lbs for LF Merlins).
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