Thanks Jeff, it looks like the surface has a insulating coating on it? If it is, then that will be the electrical barrier.
Also, the white smear around the hole on it's own and in the lower right corner of the bottom pic, is that the remanents of a thermal compound?
Maybe their design was to have the pcb in close contact with the insulated base, with some thermal compound between the pcb and base so the base could act as a heatsink for the output transistor, which is the largest device on the board.
That transistor regulates the current through the rotor winding and hence controls the output voltage.
It will generate a bit of heat, especially driving at night with headlights on which demands more rotor current. Would also be advisable to check on the thermal design of the original solid state setup.