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Here's the culprit: The sheetmetal skin of the jamb fatigued over the years, and cleanly broke loose behind the striker's washer. Now the striker is only attached to the backing plate (visible through the hole here), which isn't solidly secured to anything. In time the backing plate will pull through the jamb, creating long cracks in the process. The broken piece is marked from behind to show which side goes up, so it can be replaced in the same position.