The power-curve above is for the final development of the P56 1.5 litre engine, introduced at Monza in 1965.  An earlier version of the 3 hemisphere head was introduced at the International Trophy Race at Silverstone.  The power of this engine "increased all the way up to 11,000 rpm, and then gradually up to 11,500 when the power curve flattened out" - Tony Rudd It was Fun - and so was presumably somewhere between the two curves above.