Special Collections and a letter from the King

During one of the classes at RootsTech in Salt Lake City this year, the instructor kept emphasizing how we should not discard unlikely places for the answers to the questions we seek.
'yea, yea, yea' I thought. Sure I'm going to spend time going to out of the way places to find an arcane piece of information.

Yet, today I have found 2 off the wall places for some great information and I just had to share this one.  I was searching on google for information on the Battle of Lys [France] during the Great War [World War I].  I had recently obtained the discharge papers for my husband's grandfather and it listed that battle as one in which he had been engaged.

According to an army military website, only 500 Americans had fought in that battle.  The battle was fought from 9-27 April 1918.  In looking for additional info and any photos from that time, I came across the University of South Carolina's Special Collection of The Samuel Bloom World War I Archive. This archive contains papers of PFC Samuel Bloom from New York.

The website gives this overview of the collection:
"Overview of the archive: the archive covers the experience during and after World War I of PFC. Samuel Bloom (1895-1976).  It includes Samuel Bloom’s
 diary in the months before the war, and either diaries or the draft of a book recounting his war experienceweekly letters home (often of 8 or 10 pages) to his parents in New York; letters from the family to him (usually written by his brother Hyman) recounting family and current political events in New York; letters to him from friends, both in the army and back home; army documents; material on the Army School Detachment at the University of Montpellier in spring 1919 (including seven issues of the newspaper, theSoldier-Student); soldiers' guides for leave in Paris and elsewhere, and other guidebooks; and a group of postcards. "

 What I found most interesting, however, was this note from "Windsor Castle" signed by George and dated April 1918 to the Americans joining the war....was this note for the American soldiers going to the Battle of Lys?