Image Number 2 for United States Patent #6477875.
A twisting tumbler lock with a side bar is provided with additional levels of security by a axially reciprocable slider which is normally positioned to block operation of the side bar but is moved by a specially configured key to free operation of the side bar. This provides a third level of locking security, it also allows master keying. The unique key has a portion of a rib formed to cooperate with the slider to further allow key control.
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