I haven't had the pleasure of working on an aftermarket gov, and do not have the tool/ jig.
My great running 8n, with engine off, full throttle, measures 2 9/16 in from the flat spot in the pic, to the small arm.( trying to simulate the jig measurement, with gasket)
If the aftermarket govs small arm is too long in this dimension, seems it would give high rpm. I grinded an old welding rod to fit, then measured the rod.
Not as precise as the jig, but may be helpful.