If it is a southern gun? It may be for a bird feather, the feather could be used as a vent pick that would not wallow out the Vent hole and could also plug the Vent hole if the rifle was leaning against a tree or wagon Loaded all day for subsistence hunting/ protection. The feather plugged touch hole was thought to help keep humidity from getting to the main charge.
The "Fluff" on the feather would hold the feather in the inlay hole, and feathers are not costly so a few spares can be kept in the shot pouch if a feather was lost. Just my .02 as I am not a historian....just something I read.