It's ironic the subject of this post came up again. Since my last post, I found another cover that is addressed to the same individual while in France during WWI, some 13 years earlier (1917):

Although the postmark is weak on the front, it is clearer on the back and in this case it does show a monogram of the sender. (It also has the Tiffany & Co. marking):

These are similar covers, I know, but NOT from the same printing, as the first one I posted did not have the return address monogram.