A.K.A. the double-action sear. Considering the age of the gun I imagine it just got held up with some gunk. As you've noticed it's spring-loaded and needs to pop out every time for the trigger to catch it. Just make sure your cleaning got into the area where it rides and you should be good for another 42 years. Don't over-do oiling it either; for revolver innards I like a dusting of graphite.

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