I don't find a reference to this overprint on Scott
edited June 18
It is a pre-cancel issued in 1915. Luxembourg issued such pre-cancels from 1908-1917, and from 1919-1925. They will not be listed in Scott as they do not change the stamp. All of these pre-cancels show the country name and a two-digit number depicting the last two numbers of the year of issue
There is another, older type of precancel used in 1900. It is a single-line box with "LUXEMBOURG-VILLE" on one line, and "1900" below it inside the box.
Thanks again, Michael. I don't believe this explanation would have occurred to me. (I used to go crazy trying to identify the "overprints" on those early Belgian stamps, only gradually coming to realize that they were precancels.)