His instrument is a piston valve horn, which was the design commonly found in France and Britain during this era. Germanic countries favored horns with rotary valves and that style has become the standard for the modern instrument. But the piston valve horn was equally popular, especially for military bands. The older natural horn or hand horn which uses no valves, was still common in France up to the end of the 20th century. Compare him to two other army horn players who used piston valve horns - Adolf Adel from 1896, and the West Kent Bandsman from 1914-18.
The card was sent from Welkenraedt in Belgium's eastern Walloon region to a Captain Felix de Prat in Leganés, Spain. The simple signature with the number 65 is not clear to me, but I'd like to think there might be some significance in the choice of a bandsman postcard. Perhaps Felix was a horn player too.
Luxembourg Military Band whose musical traditions continue today. The website for the Luxembourg Army History even gives a roster of the bandmasters.
(1872-1957) served as bandleader from 1909 to 1937, and I believe he is seated in the center of the band, either the officer with sword or just to the left. This illustration from 1925 comes by way of Wikipedia and shows Mertens with his predecessors. He was also Belgian, and composed a number of marches for band and a few operettas.
For contrast here is the U.S. Marine Band from the same period before the Great War. It is arguable which band has the greater proportion of mustaches. This postcard has the more typical formal pose for military bands and shows 8 horns in a line. All appear to have rotary valves except for possibly a piston valve (1st L) and even one without valves (3rd L). This would likely indicate the country of the bandsman's musical training, as the band took on musicians from many different cultural backgrounds.
Standing in the center is William H. Santelmann (1863-1932) who served as bandleader from 1898 to 1927. He was one of many German musicians who immigrated to America and found employment in the Marine Band. His son, William F. Santelmann, continued in his father's tradition and served as director of the Marine Band from 1940 to 1955.
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