Break it on purpose
One wrong character in 240 is refused. The rest are accepted, and about three in ten of those name a plausible building somewhere else. Nothing in the format tells you. Press a character and see.
Under each character: how far a single wrong symbol there would move this code, taken as the middle of all twenty-four possibilities. Computed for this code, not quoted.
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Getting it back
The structure that hides the error also locates it. Given a rough idea where you were (a device fix, a named suburb), the wrong code's neighbourhood is searched for codes one keystroke away.
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