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Letters from a Civil War soldier, home - I'm not sure if the 'covers' as they were called were army issue, but I've now seen quite a few from soldiers to home that look like this.
This letter represents the letter that was being sent to England from King Claudius. Enclosed in the letter was Hamlet's order of execution. Obviously the letter did not make it to England with that writing, because Hamlet saved himself, but in doing so sacrificed his two friends lives. Hamlet finding letter was a turing point, because it saved his life.