A possible solution to the mystery of "what happened to the negative of the post-mortem exam?":
American Journal of Photography, Volume 14, 1893
“A Valuable Find. A hidden treasure was unearthed in Washington which promises to be a valuable addition to the relics of the war. It was discovered by J. Watson Porter, an itinerant photographer, in a house on Pennsylvania avenue, between Twelfth and Thirteenth Streets, N. W. It consists of a collection of rare photographic plates, taken from life, of Abraham Lincoln, J. Wilkes Booth, Charles Sumner, Edwin M. Stanton, Salmon P. Chase, Vice-President Henry Wilson, Horace Greeley, General U. S. Grant, General George B. McClellan, Rev. Henry Ward Beecher, Prof Agassiz, and a dozen other persons of almost equal importance in the history of the land of the Stars and Stripes. The above negative were shown a Post reporter by Mr. Porter, who picked them almost at random from a collection of hundreds of cob-web covered and dirty negatives, which, through Mr. Porter, had for the first time in more than a score of years seen the light of day.
“While this batch was being looked over Mr. Porter pulled out another box, which contained negative still more valuable. There was a copy of the old Dunkard church near which the battle of Antietam was fought. This negative was taken in 1863, before the battle. The very next plate was the only one taken from life of the hanging of Mrs. Surratt at the Washington Arsenal. There was one of the navy yard taken in 1863; another of the old Baltimore and Ohio depot, where so many stirring scenes were enacted during the trying war times; one of Pennsylvania avenue, between the Treasure building and the Capitol, as it looked in 1861, and still another of the Marine Barracks, taken in 1863, and many others equally interesting.
“That this collection could have been so many years hidden and neglected in the heart of a city like Washington is remarkable, but the collection tells its own story. The entire lot numbers between five and six thousand plates, more than half of which are likenesses of persons either celebrated or notorious at the time they were pictured, or who have gained distinction. Hundreds of them are of officers and soldiers of the armies of the Cumberland and Potomac, and others of naval officers and Federal employees of prominence.”