My son has a pre-1970 Mossberg 500C (20 gauge) with a malfunctioning safety. Squeezing the trigger causes the safety to disengage. I've had the gun fully stripped a number of times, and there is nothing obvious wrong with the safety button (it's the old metal kind), detent, safety block, upper trigger (where the trigger engages the safety block), or anything else I can find. I've noticed that the online tutorials about the 500 show a "plate" that holds the detent in place. Those must be newer models, because this one seems to be pressed directly into the receiver. Any help would be appreciated.