Some people have had the part circled in the pic break. Maybe that is what fell out.
When you stick your key into the lock at some point during rotation of the Key the nose of the Key pushes that part downwards.

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84 300D, 82 Volvo 244Gl Diesel

Thank you so very much for the reply! Didn't expect it since thought the topic was done, so nice surprise.

The bit was very small and came from inside the tumbler when I removed one of the brass pins, not from the housing. Was I not clear?

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Current fleet:

1985 Mercedes-Benz 280TE - Waiting for heart surgery.

1985 Mercedes-Benz 300TDT - Rear ended 23 September 2016 and now looking for a new home.

1979 Mercedes-Benz 300TD - Parted out.

1964 Volkswgen Beetle - Vater's since September 1968 and undergoing a restoration.

1971 Volkswagen Sunroof Squareback with F.I. - in need of full restoration.

1971 Volkswagen Squareback automatic with F.I. - Vacationing with her caretaker until he is in better health.