It seems to indicate horsepower is 1500 at low blower, looking at the chart, it seems to have a twin-speed setting, I don't know what horsepower is produced in high blower?
The idea I was thinking of was, if you had a twin-blower set-up like the F4U-1 or F6F-3, and the same horsepower as either of those two, how much of a difference would you see in speed at higher altitudes.
The chart reads out everything except horsepower altitudes.