Very cool letter. It would be interesting to learn the rest of the story behind that Ithaca's trip to England and the Birmingham proof marks. Here is a link to posts on trapshooters.com of another John Boa Ithaca 6E SBT made in 1919. The 6E has an eagle instead of a beaver on the bottom, but it still has the made in U.S.A engraving. There are links from Dr. Drew to info on John Boa. One is a link to an image in Oct. 1916 to Boa with his Ithaca in Sporting Life, but the link isn't working. It is probably your gun.
https://www.trapshooters.com/threads...6e-sbt.832915/