This is an alto Helicon in Eb made by Jerome, Paris.   Jerome apparently was an exporter of brass instruments- but not much else is known about this company. The bell inscribed:

This horn was found in Thailand 1997.   Helicons such as this were often brought to Indochina by  French  Colonial Bands - but this one may have been imported for use in the Royal Thai Army Band. Helicon shaped brass instruments were regularly used for marching bands- and were sometimes employed in mounted bands.  It remains in good working condition.