The factory prefix table logs KH6XX as KH6 and KP4XX as KP4 here ok.
Go to tools/setup/prefix table.
Look for a KH6 record like in the attached screen shot.
Click the binoculars and enter KH6XX for the callsign. The record that is associated with it will be displayed.
What is the factory prefix table not doing for you? Since we integrated the CTY files, most of the callsigns that don't follow the usual prefix scenario are accounted for.
Tnx & 73,
Dennis WN4AZY