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Must they necessarily open exactly 50 envelopes? Perhaps a prisoner finds their number, then continues to open envelopes until they reach 50 or until they find their number + 1. This would indicate to the person next to them where their number was.
I should have been clearer that the envelopes are all returned to the exact same initial state for each prisoner. All closed, each in the same physical state and position, each containing the same prisoner number. They can keep opening more, but they can't use that information.
