However, we can infer their contents from the following:
* Hamlet's suspicion: Hamlet suspects that the letters are not what they seem, and he fears they might be a ploy to get him killed. He even suspects the letters may call for his immediate execution.
* The King's intentions: We know Claudius is trying to get rid of Hamlet, and he is already plotting to have him killed. This makes it highly likely that the letters contain instructions for Hamlet's death.
* Hamlet's actions: Hamlet, in a moment of desperation, takes the letters from Rosencrantz and Guildenstern, and changes their contents to order the execution of the two men instead.
Therefore, we can reasonably conclude that the original letters contained instructions for Hamlet's execution, and possibly even the methods of carrying it out.
It's important to note that the content of the letters is left deliberately ambiguous by Shakespeare, adding to the intrigue and suspense of the play.