What is the groove you mention in the governor shaft? I've repaired or rebuilt many DB2 pumps, but I'm not sure what you're referring to? I don't recall any common wear areas associated with the governor getting grooves in them.
As far as setting the high-limit for R.P.M., doesn't your pump have an external adjuster screw with a jam nut and lockwire? I'm a little foggy on the DB2 when installed on the I.H. engine; most I've worked on were on industrial and forestry equipment.