Not musket, double barreled flintlock muzzle loading shotgun. It would have a ramrod and slings were quite common on guns like these.
It does appear to be a working gun and not a tourist item. You may have a nice find there. Value impossible to judge as is shooting condition.
Look for a true muzzle loading expert/gunsmith for better info.
Let us know what you learn.
BTW: Do not attemp to shoot until it has been checked out by an expert.