The following is taken from the NARA descriptive pamphlet for M1899, which can be downloaded here.
During Allied air raids on Frankfurt, Germany, in October 1944, the Hesse family, led by Prince Wolfgang of Hesse, decided to hide ancestral and personal property at the family castle, Schloss Friedrichshof at Kronberg (Kronberg Castle). A large box was constructed into which were placed the jewels of the House of Hesse, the property belonging to Prince Richard and his wife, Prince Cristoph and his wife, and Prince Wolfgang and his wife, as well as jewels and other property belonging to Princess Margarethe, the dowager Landgrafin of Hesse. A hole was dug in the basement of Kronberg Castle. Once the box was placed in this hole, it was covered with concrete. An inventory of the contents of the box was created.
Sometime between April 10 and April 20, 1945, Kronberg Castle and its adjoining buildings were occupied by American forces. The castle was requisitioned for use as an officer’s club, and the establishment was turned over to the Mess Section, HQ Command, United Stated Forces European Theater. Captain Kathleen B. Nash was appointed “Mess Section” representative from June 1945 until February 1, 1946, and was essentially the officer in charge of Kronberg Castle.
Captain Nash discovered the cache of jewels and property in November 1945. The box was unearthed, and the contents placed in her sleeping quarters. The box contained pieces of jewelry, loose gemstones, silver, tableware, and books. The value of these pieces was subsequently placed at over $2.5 million.
Major David F. Watson and Colonel Jack W. Durant were frequent visitors to the officer's club and with Captain Nash conspired on how to keep the property and smuggle most of it into the United States. Meanwhile, members of the Hesse family were informed of discovery of the hiding place. When the family and their representatives contacted Captain Nash on several occasions to seek assurances that their property would be returned, she promised them that it would be once the castle was vacated.
Nash, Durant, and Watson were apprehended by military authorities after they returned to the United States and tried to fence the jewels. Nash and Durant were married in a successful attempt to prevent Nash from having to testify against her new husband. The three were charged with violating the following articles of war: 61 (Absent Without Leave), 93 (Larceny), 94 (Frauds Against the Government), 295 (Conduct Unbecoming an Officer and a Gentleman) and 96 (Bringing Discredit Upon the Military Service). Nash, Durant, and Watson were tried in separate court-martial proceedings held in Frankfurt, Germany, and Durant's trial was carried over to Washington, DC, between August 1946 and April 1947. Each was found guilty of at least some of the specified charges, and all of them were sent to prison.