26.5,
Just throwing in a formula in here. Maybe it might help.
IMN = TAS/LSS
IMN = Indicated Mach No.
TAS = True Airspeed
LSS = Local Speed of Sound
LSS is a function of temperature. The lower the temperature (higher altitudes), the lower the LSS. We know that if we climb at constant IAS, the TAS will start increasing. Using the formula above, we can see that to maintain a constant IMN, TAS has to decrease as we climb higher (because LSS decreases with altitude). As a result, IAS decreases as we climb at constant IMN. If you imagine this IMN to be your barber pole (Mmo) and it is being communicated to you on your airspeed indicator (which displays IAS), you will find the barber pole decreasing to lower IAS as you climb. And as a result, you will have to reduce your IAS as you climb (at constant IMN) because the barber pole gets lower along the way.
As JT mentioned, at lower altitudes Vmo is the limiting speed and at a 'certain altitude' upwards, Mmo is the limiting one. That is why we climb at a constant IAS until that 'certain altitude' after which we climb at constant IMN.
Now if you imagine that 'certain altitude' as 15,000' to 20,000' in the example you gave, you can see why the barber pole starts decreasing. That is a pretty low Mmo I think. But then again it's a prop. Hope I have made the picture clearer for you.
9V.