Not sure if this "tinkled down" to CC engines, but on the full-size Farmalls, the torque/horsepower curves were optimized for the upper end of the RPM range.
As I sort of very vaguely kind of recall, the large tractors would be run up to full throttle (high idle) and then the dynamometer would load the engine till the tachometer would be on the rated PTO speed mark, and observe Horsepower reading on the dynamometer.
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