Turboprop Mmo

[pattern_is_full]

It could just be that Vmo/Mmo for the MIII is simply a result of FAA paperwork and math.

I don't know if the MIII is certified under Part 23 or Part 25 - but the critical phrasing involving definition of Vmo/Mmo is the same for both - and note that it is a derived speed, defined simply as being "sufficiently below VD /MD (design diving speeds)...to make it highly improbable that the latter speeds will be inadvertently exceeded in operations.":
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FAR Part 23

§ 23.1505 Airspeed limitations.
(a) The never-exceed speed V NE must be established so that it is—

(1) Not less than 0.9 times the minimum value of V D allowed under § 23.335; and

(2) Not more than the lesser of—

(i) 0.9 V D established under § 23.335; or

(ii) 0.9 times the maximum speed shown under § 23.251.

(b) The maximum structural cruising speed V NO must be established so that it is—

(1) Not less than the minimum value of V C allowed under § 23.335; and

(2) Not more than the lesser of—

(i) V C established under § 23.335; or

(ii) 0.89 V NE established under paragraph (a) of this section.

(c)(1) Paragraphs (a) and (b) of this section do not apply to turbine airplanes or to airplanes for which a design diving speed VD /MD is established under § 23.335(b)(4). For those airplanes, a maximum operating limit speed (VMO /MMO airspeed or Mach number, whichever is critical at a particular altitude) must be established as a speed that may not be deliberately exceeded in any regime of flight (climb, cruise, or descent) unless a higher speed is authorized for flight test or pilot training operations.

(2) VMO /MMO must be established so that it is not greater than the design cruising speed VC /MC and so that it is sufficiently below VD /MD , or VDF /MDF for jets, and the maximum speed shown under § 23.251 to make it highly improbable that the latter speeds will be inadvertently exceeded in operations.
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I.E. there is no particular reason (aerodynamically) to set the Mmo at 0.52 - except to mathematically make it "sufficiently below" VD/MD to prevent inadvertent exceeding of those speeds.

What's "sufficiently below?" Who knows? 5%? 15%? Plug in an educated engineering guess.