64iu0s

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It turns out all the stationery enthusiasts have been locked away in the prison of social media on their phones. We've finally found them, but where's the key to unlock the cell door? At that moment, the prison guard was changing shifts and handed a note to the colleague taking over, saying, "I've left the key here." What could they possibly mean?

What's the code? (Hint: The word contains three letters.)