Rolf: it looks like the lock is going to hide it, so go ahead and inlet the barrel. If after doing so, and the lock you can still see some evidence of the blemish, mix up some AccraGlas (from Brownell's) add some dark stain, blow out all of the dust, apply drops of the epoxy until it is filled to overflowing, and then apply gently, a piece of 2" wide packing tape over the filler. The tape will hold the liquid epoxy in the pit and when it cures, and you dress if off down to the wood, you'll have a dark beauty mark, which in English walnut, is appropriate. A lot of original European guns, when made of highly figured wood, have plugs and filler.