to make some pickguards can emit a chemical that will corrode metal. Metal like your pickups and frets for instance! This is condition gets much worse if the guitar is left in it's case for a long time...a good reason to actually PLAY those vintage guitars. Today's L5 is about a 5 on a scale of 1-10 in terms of damage. It sat in it's case for too long before Grampa handed it down.
You can see the bits of rust and corrosion everywhere. When it's gotten to this stage,there will be some loss of the gold plating but thankfully,all the electronics are fully functional.