Assuming the gap is too small for a spliced-in piece, and too big to not be noticed:
I have some shellac sticks in various colors, bought at a woodworking store. Heat until melt, smooth in with hot spatula. Numerous colors available. But ... I don't know how well this would take the stock finish, or how well it would hold up to repeated blows from a buttplate.
Also tried epoxy mixed with sanding dust from other parts of the stock. Never seemed quite right, always too light a shade. Tried dyed Accra-Glass, but most critical eyes who saw it said: "That looks like dyed Accra Glass, but a nice fill-in!"
Tried pounding in small slivers of wood mixed with adhesive, which look OK except the adhesive (Elmer's glue, Gorilla glue, white glue) is always is lighter colored.
Other than a 10 year apprecticeship to a master stockmaker, suggestions welcome.
I