I can not tell you what model your rifle-musket is, but I can tell you is was made in Belgium, by O.P. Drissen, Liege. I have a similarly marked patt. 1853 Enfield, in .58 caliber rather than .577. Yours is most likely a French pattern musket / rifle that may have been modified in its configuration.
Do you know if this would have been a Confederate Musket? How would I tell. I am looking to sell this musket, How would I even know what to sell it for? Thanks so much for the information.