Gene, there's a letter from Watson to John Surratt in The Lincoln Assassination: The Evidence.
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Envelope: Postmark New York __ 19, Address; Mr. John H. Surratt, Washington, D.C.
New York, March 19, 1865
Mr. J.H. Surratt
Dear Sir,
I would like to see you on important business, if you can spare the time to come on to New York. Please telegraph me immediately on the reception of this, whether you can come or not & oblige.