There is a hole that passes all the way through. The key has a large tab that sticks out from the stem [(pic)](https://www.turbosquid.com/3d-models/old-key-3d-1220230). The tab goes in below the hole, and as you turn the key the tab interacts with the mechanism, unlocking the door. The mechanism can thus occupy the cylinder swept by the tab, all but a little bit of which is not in the keyway.
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