It would be the second time the special counsel has notified the former president that he is likely to face indictment, this time in connection with the criminal investigation of the Capitol attack.
The short, unpublished letter had been in the same private collection for at least a century before it was acquired by the Raab Collection in Pennsylvania earlier this year.
Researchers are sorting through a centuries old cache of undelivered mail that gives a vivid picture of private lives and international trade in an age of rising empires.