As to your wonderful card, Linus, it is odd how often a story such as yours is told. We do find things when we least expect it in places we had no logical business thinking we'd find what we did.
Is that cover in your collection? If so, where did you find it?
I must wait for it to arrive, but it is mine. Closed today on eBay
with this listing title, "1918 US Army Expeditionary Forces Italy Censored Cover to APO France Xmas Seal" and I fought off one competitor. That helped me figure that while the open was high, it was reasonable.
[Note: until the listing disappears, put the title in the search box and search. Then when likely nothing is found, look to the left of the page, scroll down and then tick "completed listings" and it will appear. And yes, you will see the price.]
I would think it would be considered rare usage of WX13 on cover.
I must agree, in fact I never thought about Christmas seals used by AEF personnel. When I looked at the search return, I was expecting to see a different seal given the 1918 date. I was knocked-over surprised and since I happen to collect WX11-14, I decided to try for it as I believe it is a fantastically unexpected usage.
Later in this thread, I will post some of my other WX11-13 stamps and on mail usages. I own no WX14 yet.
I backed in to my interest in these WX11-14. I collect parcel post and once in a great while I would find a card with a Q-1 mailed in 1913 with a WX13. I liked the design of Santa at travel and my interest broadened. I got my mint WX11 from an APS sales booklet after losing one in a Harmer-Schau Auction a couple of years ago.
Speaking of AEF, there are two recorded usages of parcel post stamps from AEF personnel in Europe, neither of the three cent (Q3) stamped letters has a Christmas seal.