Hugh
In the link I sent on 17 April at 12:56 p.m. is a Memorandum from Prisoner of War Records on which I based the statement you are asking about. That memorandum reads: "He was detected Jany 17/64 as a Deserter from the U.S.A. as of the above name and organization [Samuel Merritt, 3d Md. Cav] while a Prisoner of war at Fort Delaware, Del under name of Maddocks 42 Va and is reported date of Jany 19/64 as sent to Washington D.C. for trial Jan. 17 - 64" with the copyist's note "See Rec of Ft. Del." Since the copyist indicated the information was taken from the records of Ft. Delaware, it seems to me that the memorandum can be read that Mattox was sent from Ft. Delaware to Washington, D.C. on 17 Jan 64 -- with the report of that transfer made on 19 Jan 64. That's a dating correction from my earlier message to you.

Another way to read the Memorandum could be this: "He was detected [on] Jany 17/64 [at some unspecified located] as a Deserter from the U.S.A as of the above name and organization [Samuel Merrit, 3d Md. Cav.] [who while] a Prisoner of war at Fort Delaware, Del [during an unpsecified previous period was listed] under name of Maddocks 42 Va and is reported [on the] date of Jany 19/64 as [having been] sent [from an unspecified location] to Washington D.C. for trial Jan. 17 - 64. The question I have about this interpretation is linked to the copyist's notation to refer to or check the records of Ft. Delaware. If Mattox deserted from Baltimore, having left Ft. Delaware behind for good, why would anyone at Ft. Delaware be interested at all in Mattox's post desertion fate, at least to the extent to record Mattox's subsequent status, unless someone at Fort Delware had responsibility for Mattox at some point after Mattox's desertion from the 3d Md. and needed to account for Mattox's status?