Not related to prop tip speed, nor prop rpm, so far as I know.
Recall the F.27Mark600 (same basic airframe as the F.50) had a Vmo of 224 knots.
Yet the Fairchild FH227B, with the larger diameter props (same engines, RDa7's) had a Vmo of 250 knots.
However, the FH227B had an elevator trailing edge 'wedge' fitted (Fokker F.27 did not), so suspect control surface flutter was the concern with the slower designs. Visited the Fairchild factory years ago to have the larger diameter props fitted (for high altitude performance considerations), and when the change was made, off came the elevators to have the wedge fitted.
Acording to the old timers there, was for elevator flutter concerns at the higher Vmo.
And, they had the flight test data to back this up.
Interesting folks.