[ACE-OF-ACES]

I disagree, although the angle used is likely to have a small margin of error through out the climb but the pilot is going to aim for the angle at which the plane climbs best. Every plane has an optimum angle of climb, and its based on a number of factors. I can't follow the logic that they climbed at steeper angles because they could, when by design the plane climbs more efficiently at its best climb angle.

I think you might be confusing 'best clmib speed' with 'best climb angle'?

Props tend to have a best climb speed that does not change much if any during a standard rate of climb test

That is to say the speed remains constant during the climb from sea level to max altitude

What does change..

And must change!

If the speed is constant is the climb angle

For example

At lower altitudes the best climb speed for the 38 is around 165mph

At higher altitudes the best climb speed for the 38 is around 165mph

What changes from sea level to 30kft is the pitch angle (and yes AoA angle)

You start of climbing at a steep angle at lower altitudes and end up climbing at a very shallow angle at higher altitudes.

Thus pilots were very use to the idea of changing the climb angel as they climbed

An experienced pilot doesn't blow all his energy trying to get the most height in the shortest time, he does it so he can get the most height for the energy he has. That's going to be at the best climb angle which coincides with the 165 IAS speed at a WEP setting, in this case.

It depends

If your goal is to try an improve the time to clmib values than you would convert your excess energy in such a way as to minimise the time to alt

If that is not your goal than you are wanting to keep some excess energy in reserve for some 'other' reason.

In short you and I could come up with dozens of combat reason to keep some in reserve, but if your goal is that of that test than they were trying to see how they could improve the time to clmib values by zooming as high as they could in the least amount of time.

Its also a rather uniform test, with two planes climbing at the same angles and duplicated twice to determine the results of a climb from a fast cruise, and a climb from a zoom.

The same 'unknown' angle to be more accurate