This is a small 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.

P.S.   This neat little horn is now owned by Paul Sherman in NY (see pic)
he determined that it is actually a small Eb bass- not an alto (oops).  Paul is a collector of real and reproduction Medieval and Renaissance instruments.  Scroll way down to see photo of him playing his 1810 bass serpent  made in Lyon.