Not going into the details because it's kinda hard to explain, but in essence, the top surface of the slots in the stock and the bottom surface of the bbl lugs are what forces the bbl and stock together when the key is pushed in.
As was said, after an accurate layout is made, holes are drilled w/in the confines of the slot layout and a smaller wedge is heated to red and pushed into the holes and burnt through on both sides. If escutcheons are used, I favor the bottom side of the slots in the stock seeing the escutcheon will cover the gap.
A needle file is then used to file the upper surface of the slots in the stock for a proper press fit w/ the lower surface of the slot in the bbl lug which isn't touched at all.
An accurate layout of the slots in the wood is the most difficult part of installing keys, especially the upper surface and the rest is easy.....Fred