So, if you didn't use it, what caused the "tempering" and discoloration?
Was the external power thru the rear jack panel or something just tied in and run that way?
Because if say a battery charger was used, and the output of the regulator was never bypassed - you'd see a similar effect from the power dissipation the built in regulator offered to provide extra filtering and regulation - only it (any voltage variance slightly higher than the "Sense" loop the internal regulator used - as dissipated in the input - which was on it's output - instead.
Ok, the reason why this comes up was from experiences in replacing those Uniden - style 3-pin connectors that used a built in detent that used a "Cherry switch" - when you plugged something into the external power jack it automatically turned off the lines from the internal power supply feed - but not always! The switch and or internal power supply - if left plugged into the wall - kept the radio powered all the time and eventually the owner began hearing of complaints of a hum in his audio and eventually the internal regulator failed - and he couldn't understand why...