When my SD temp read below 80C at times and close to 100C at other times, it was an old, slow reacting thermostat.
When my SE temp read below 80C at times and close to 100C at other times, it was a stuck reservoir cap.
When my SDL temp would go to 100C and beyond all the time, it was cracks in the head
The M103 thermostat cover has a ridge that clamps down on the o-ring on the circumference of the thermostat. A rubber mallet won't do much unless you hit upwards on the snout. Or stick a long screwdriver in the snout and pry. I don't know what precautions to take if you have a plastic thermostat cover.