On a turboprop type, the Mach number part of the limitation is almost always associated with prop tip speeds.
Simply, Mach numbers of the order of 0.50 are too low to be having an effect on the airframe - interesting stuff really only starts to happen in aerodynamic terms when the Mach number gets to the 0.80-ish values. So there's no expectation of wing, tail, etc having an issue at 0.50.
BUT
The prop tips will be doing something like 0.80 or higher when at high RPM and 0.50-ish aircraft speed. They are the only part of the airframe seeing transonic behaviour at those aircraft speeds. So they are the part almost always first affected by Mach effects, and thus the part which drives any Mach limit.
Very surprising. What about Va, control flutter etc. I have also flown a few of Olive Anne Beech's products (excellent aircraft) and I cannot understand how the prop tips on that type could ever reach transonic speeds. A C-185 with a 3 blade prop in winter is a different story. Are you saying that if I feather the props I can go ahead and do a rapid descent above the Barber (not Barbers) pole safely?