After the final defeat of Napoleon, Europe was awash with surplus muskets owned by armies that could not afford a new generation of percussion weapons. So many countries tried to squeeze the maximum usable life out of what was probably the most widespread musket pattern ever. They were converted to percussion en masse, and some even rifled. I have in my cellar a musket barrel that had been rifled, then bored out (again!) and shortened to make a shotgun. The traces of rifling only became visible at the breech end (!!!) when I was able to remove the breech plug.
So look very carefully at the breech area, the lock, and the lock morticing. It may be a percussionized/rifled example that has been reconverted back to flintlock. If it is NOT a reconversion, then it is a rare beast.
And a word of caution from one who has been fooled by photos: get it in writing that the rifling goes right down to the breech.