Literally. This has just arrived in the post this morning from Peter Harvey
It is franked 28th December 1915 FPO H5, which Proud lists as in use in the Ypres area at this time. The censor mark looks like CM3/22 which I have previously recorded as from the Queen's Own Yorkshire Dragoons, V Corps (ex AK material). It also bears a French "T" To Pay mark and two US 2c postage due stamps.
Peter suggests that the sender was probably an American volunteer writing to a friend at home.
On Ancestry, I have found that the addressee, McKee D Morrison, by then a private in the US 107th Infantry, National Guard 27th Division, sailed on the USS Susquehanna from Newport News, Virginia on 9th May 1918. It appears he was killed in action on 29th September 1918 at Vendhuile in France
A very interesting cover - I am always amazed by the amount of personal information you are able to find on Ancestry, FWR and other sites/ records - keep it up!
Mike :)
Very Nice cover
Chris