A frizzen from a flintlock of some kind, but no way to say which one. There were other rifles used by the army of the period as well as muskets which were more common. Could also be from a civilian arm.
I believe they were issue weapons of some of the federal troops in the Seminole wars in Florida. Not sure if they would have been in hand in 1817 tho...I have one on my wall from a battle on New Years Eve 1835.