Search for pictures of an 1863 Remington Zouave rifle and see if it looks like that. If it is, the Zouave is a good shooter if it is in safe condition. You ought to have the musket checked out by a reputable gunsmith.
Some imported Zouaves were marked as you say.
But, without pictures, we are just stabbing in the dark. Also, if it is a Zouave, then it is likely a .58 caliber.