I searched Google ("tompion" "muzzle" "musket") and found a bunch of hits - you will notice the configuration is slightly different. The closest one I found was this 1853 Enfield tompion.
So - looks like it might not be a tompion (as other members suggested). FWIW - What is a Tompion? - this link discusses its use (...Back in the day of flintlock firearms, keeping weapons operating in the rain, snow and mud...A tompion is simply a cylinder, usually made of wood, which is pushed into the muzzle end of the barrel...)