The firm Couesnon was established in Paris in 1882 by Amédée August Couesnon, the son-in-law of Gautrot aîné who took over the directorship of Gautrot-Durand et Cie. By 1883 the business was styled “Couesnon, Gautrot, et Ci Cie.” and from 1888 onwards as “Couesnon et Cie.” In 1890 it humbly advertised as “manufacture d’insts. de musique la plus importante au monde.” In 1896 it reported a workforce of 200 at the Chateau-Thierry factory. By  1911 the firm was making every kind of musical instrument, employing a workforce of 1000 at eight factories, and was then the largest single musical instrument manufacturer in the world.  In 1914 he factory at Chahteau-Thierry was making annually about 50,000 brass instruments, and about 10,000 at Garennes.  By 1925 claimed to have made over two million musical instruments at their six factories, however by 1927 when the firm celebrated its cententary, the workforce had dwindeled to 200. By 1931 it became known as “Couesnon S.A.”
     Among their predecesseurs et maisons réunies were the wind instrument making firms of Association Général des Ouvriers, Feuillet, Gautrot aîné, Guichard, Lecompte, Massin & Thibouville, Triébert, Tulou, and others. The former trade names were often still used after the acquired firms ceased to exist. A catalog of 1913 states “comme par le passé, nos instruments porteront, suivant leurs diverses catégories, nos marques de fabrique Gautrot aîné, Gautrot-Marquet, Tulou, Triébert. Nous les livrons également sans marque, à la volunté de l’acheteur” [as in the past according to category our instruments will be stamped with the proprietary marks of  Gautrot aîné, Gautrot-Marquet, Tulou, Triébert.  On request , we can also supply them unmarked.]  A 1913 catalog gives the following key to the quality range of their products: “Modèle Monopole” the most expensive, “Armée BN,”, “Armée GNM”, “National BO”, “Universal CGO”,  with Modèle SO” as the cheapest.

The horn shown below is a full double horn in F and B-flat with ascending third valve and was introduced ca. 1934. It is in generally good cosmetic condition but the valves are heavily worn and leaking badly.

It isn't clear from the above description whether model 373 applies to this full double horn or to the the compensating version, or to both.