Excellent explanations sir. Yeah I used to knock off stuff with a file after the heat treating and before final milling when I was making parts. Most of my machine work was from when I was designing a one-off machine or proof-of-concept. I learned OJT under some crusty ol coots who could do that stuff with their eyes closed.
I don't have much of an issue with the external fit & finish except that barrel. I wa just surprised at the rough spots inside - some of them on sliding surfaces which I why I eyeballed them and stoned a few surfaces.
I fell in love with the first Marlin I held - the rifle I handled immediately before it was a Winchester. I could immediately see the advantage of a side eject and the Winchester had horrible fit and finish - this was 1974 - and the wood looked plastic, the lever rattled when you shook the rifle and it was apparent from just handling the Marlin was a product of superior workmanship. I think that long-ago glow is the reason I was a little disappointed in the interior finish.
The important thing is the gun shoots as well as I think it should and feels solid.