With help from Lars Böttger, author of the groundbreaking Handbuch zur Besetzung von Luxemburg im 1. Weltkrieg 1914-1918 und der alliierten Besetzung, I’ve begun to better appreciate Luxembourgian WW1 postal history.
Here a German soldier from Leibgarde 115—an elite unit of the German army—has used his soldier’s brief cachet to cancel the 5c Arms postal card imprint, while the Bettborn post office has applied its bridge-and-bar cancel dated 18 August 1914 to the card. The card is addressed to Darmstadt, Germany, with the writer also showing Bettborn as his location on the message side of the card.
Böttger comments that the card shows the close cooperation between the Luxembourg Post and the German Army. The card is also unusual in this respect. The soldier could have written his message on a stampless fieldpost card or inscribed this card as fieldpost; instead, he chose to send the card canceled with his soldier’s brief cachet without fieldpost inscription.
The long, colorful history of the German Leibgarde itself is an interesting story. Relevant here is the German Wikipedia Leibgarde entry, which notes that at the beginning of World War 1 in August 1914
[d]as Regiment bestand aus drei Bataillone mit je vier Kompanien, dazu ab 1913 je Bataillon eine Maschinengewehr-Kompanie. Nachdem am 1. August 1914 der Mobilmachungsbefehl eingegangen war, stand das Regiment am fünften Mobilmachungstag in einer Stärke von 83 Offizieren, 3305 Unteroffizieren und Mannschaften sowie 240 Pferden abmarschbereit.
At page 93 of the Böttger handbook, two other S.B. cachets are shown, both in violet, with dates of 26 August and 31 August, 1914.
There is, as another Luxembourg postal historian Hans-Ulrich Doose wrote a couple years ago in Le Moniteur, “still plenty to be discovered." So true!