Hi All, the mechanism of the shock-cooling is a little complicated to decide.
At first, you would think it was due entirely to the cold high altitude air cooling the cylinders fins from the outside.... However that has remained constant even whilst under power.
The only real change (dT/dt.) has been to the internal surfaces of the cylinders having their source of heat (fire) removed and replaced by a cooling flow of non-combusted gas.
Also the metal mass of each cylinder / cylinder head contains areas that have different temperatures when in normal ops.... The cylinder walls are subject to the heat of combustion; The Inlet Valve area has the very cool inlet fuel/air mixture, with the Exhaust Valve in the hot exhaust gas flow.
So the temperature gradients are quite high in these areas. Any sudden attempt to bring them back to a uniform temperature is likely to result in shock-cooling, and not necessarily in the areas of the fins cooled by the airstream.
Just as an aside,,, In the motor-cycle world, Racers tend to run their fuel mixtures for max power, whereas owners of Concours machines run richer mixtures to prevent heat discolouration or rusting of the exhaust pipes.!
Pete