Different reduction ratio, different prop diameter I'm sure
My point is perhaps a little misunderstood.
The Merlin XX, for example, was used in,
Beaufighter II
Defiant II
Halifax I/V
Hurricane II/IV
Lancaster I/III
Mosquito I/II/IV/VI
The Merlin in the Mosquito was actually designated as a "21", but the only difference to the "XX" was the coolant flow direction was reversed.
Its rating was 1480 horsepower at 3000RPM. Critical altitude in low blower 6,000 and 12,250 in high.
There was a great disparity in speed between the Defiant and Mosquito.
It is extremely difficult to compare the performance of each of the aircraft at identical power settings, as the flight manuals of the day didn't go into such matters in depth.