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Maybe this picture will help you visualize things a little?
For the moment, ignore the top drawing and study the bottom one.
The lock has a arm that sticks out away from the lockplate that is a a part of its sear. The sear holds and releases the hammer by engaging notches in the lock tumbler.
When things are installed into the stock, the locks sear arm is located where the arrow is pointing.
Notice the angular line that represents the front trigger blade in front of the (rear) set triggers trigger blade?
This drawing is showing the front trigger blade like it would be if a finger was lightly pulling the front trigger. (Normally, the front trigger would be slightly forward and its blade would be lower.)
That rear trigger blade is shown in the "uncocked" or "unset" position with its spring holding the blade up. That's the way it normally is and it usually is very close to the lock sear arm.
If the set trigger assembly is installed to deeply in the stock, that rear triggers blade will push the sear arm up far enough that the lock sear can't engage the full **** notch in the tumbler.
If that happens, the lock cannot be set to full ****.
If a spacer or shim is placed between the trigger plate and the wood in the bottom of the slot (mortise) in the stock, the whole trigger assembly will move down.
This moves the top of the rear triggers blade down, away from the sear arm.
With nothing now stopping the sear arm from moving down, it can now engage the full **** notch in the tumbler and the lock will remain cocked.
Did this make sense?
If it did, you can understand why filing some material off of the top of that rear triggers blade will achieve the same thing.
The blade will no longer be pressing upward on the sear arm.
About this "tightening the lock screws released the hammer" idea:
More than a few locks fit somewhat loosely in the lock mortise. When they are slipping into place, they often will **** or tilt slightly and the locks sear will be higher than it normally is.
As the lock screws tighten down, the lock hits the bottom of the mortise and begins to "square up" to a normal position.
As it "squares" or straightens up, the sear arm moves slightly downward.
If one of the trigger blades is in the way, further tightening of the lock screws can cause it to move so far that it suddenly releases the tumbler and hammer so the hammer falls.