Dammit
john_tullamarine, a person goes on leave, finds an Internet cafe, and you grab all the good ones
J_T is right (as usual) about the Barber Pole increasing at lower levels due to the EAS limit, which is a structural consideration. With increasing altitude, the CAS for a given EAS will increase.
Mmo will be imposed at higher levels due to the fastest moving part of the aircraft reaching critical mach numbers (Note that I said critical Mach numbers, not Mcrit, some exceedance of Mcrit can be tolerated). For the Propeller aircraft, the fastest moving part is the propeller tip, and this will be the first part of the aircraft to reach critical Mach numbers. Thus, even though you may not have a Mach Meter, the Barber Pole will reduce with increasing height to observe the Mmo.
Even much older aircraft like the F27 had one Vno/Vne up to 20000 feet, and then another up to 25000 feet. If you did a quick comparison, they were the same Mach Number, a propeller induced Mmo limitation.
Regards,
Old Smokey