On the 24th of september 1905 Monsieur Sablon Chef, de Musique au 1e Etranger, received in Meaux France, a postcard sent from Sidi Bel Abbes Algeria.

The image at the front of the postcard is related to Monsieur Sablon his function of that of Chef de Musique since it contains the program of a concert to be given on Friday the 8th of september 1905.
Most likely this concert was conducted, in the absence of Monsieur Sablon, by his Sous Chef Trévillot.
Potentially Sous/chef Trevillot could be the one who wrote the card.

From examples of similar postcards shown in the book by J. Gandini (a) about postcards of the French Foreign Legion and examples found on the internet, predominantly cards offered for sale in online auctions like Delcampe, it can be concluded that for each concert a special card was made with an unique illustration.
The exact purpose or objective of these cards is not clear, but most likely they were made to either promote the concert, as a program for the concert or as a souvenir. Knowing the Legion always keen to raise some money, I would not be surprised if they were sold at the concerts.
They seem to be sought after by collectors because prices are always relatively high.
Following two examples of programs of the concerts held on 29-06-1905 and 09-07-1905 show the name of “Le chef de musique Ch. Sablon”

Programme de Jeudi
Le chef de musique Ch. Sablon

Programme du Dimanche
Le chef de musique Ch. Sablon
The concerts were also announced in newspapers as this example from the 1st of July 1905 from L’Eveil shows.

Chef de Musique Ch. Sablon
Information in old newspapers, found using Gallica, indicate that Ch. Sablon became chef de musique of the 1er RE as per the 4th of June 1902. M. Salomé was his predecessor.

On the following website https://military-photos.com/1rei.htm even a photograph of Sablon can be found.
The photo is a detail of a larger photo taken in the second semestre of 1905 showing:
l’état major du 1er régiment de la Légion en Algérie, probablement à Sidi bel Abbes, dépot du régiment.

Il porte les palmes académiques, distinction rare à la Légion…”
Ch. Sablon stayed in this function until the beginning of 1907.

(a)
La Legion a travers les Cartes Postales 1900 – 1962
GANDINI, Jacques
ISBN : 2-906431-36-2 Date de parution : 1998
C’est un essai d’inventaire illustré des cartes anciennes et modernes (jusqu’en 1962)
connues à ce jour sur le sujet
In GANDINI’s book are on page 17, six versions of the Programmes des Concerts du 1er Étranger, Sidi-Bel-Abbes shown.
Mentioned is that no reference to an editor is given on the cards.
Further that it is know that more versions of these cards exist than the six shown.
Shown are the Programmes held on following dates:
23-12-1904
30-03-1905
09-07-1905
18-02-1906
22-04-1906
and
Orchestre du Vendredi 30-03-1906.
All of them show as the Chef de Musique Ch. Sablon.