I bought a Vicon disc mower for cheap because the metal was broke right under the gearbox. When I called Vicon to ask some questions, the tech asked the model number, length etc. Next sentence he told me the problem....someone removed the restrictor because the cutting arm came down too slow (for Mr. farmer).
I rebuilt the broken frame, made a restrictor and am still using it today.
Vicon wanted about $60 for their restrictor fitting.