Max opens the secret panel in his front door. The coin slot can be seen on the wall at right.
Front door of Maxwell Smart's apartment.
Instead of unlocking the door itself, inserting the key opens a secret panel in the door containing an elaborate mechano-electrical apparatus. After operating various switches thereon, a coin is inserted in a nearby wall slot causing the door to open.
[Episode #57: "Pussycats Galore".]