Why does the trigger guard need filing before it fits into the stock?
The trigger guard is an investment casting that comes back from the foundry bent and twisted, so it needs filing and straightening to fit the stock properly. The edges that go into the wood also need filing because the way the part is molded gives it draft in the wrong direction, meaning the area entering the wood first is actually the largest rather than the smallest, so it must be filed down and given a bit of backdraft, along with removing flash left from the mold seam.