That's a super narrative , Kutter. The pictures help much, too. I have a job to do almost exactly like that on a Remington hammergun that had a bad chip lost right behind the upper tang. I let in a piece of walnut like you did, then re=inletted the rear of the tang into it. Now, it needs coloring. I guess I'm fortunate that I've removed all finish from the wood, and will be redoing the entire buttstock and forend. All I'll have to do is blend the two pieces into each other, then apply finish over all at the same time.
Your post will be a great help. Thanks.
