It appears that you are refinishing an original or an already used gun. There is no curl in the wood to contend with. Your best course would be to flatten the area and use dead straight grain maple to replace the lost chip. Stain and put a coat of finish on the gun and then use a darkening agent, bone black, Trans Tint dye or Mike's can of Rustolium to further darken the joint area. Feather out the darkening agent into your overall finish and then continue with your refinishing. When done the joint will likely be invisible.
Ron