From your original post I was only thinking of the peening being done on the hook for the purpose of pushing the bbls back against the face (would much prefer to go the shim route myself). I presume they peened the action flats to close the slot for a tighter fit with the bbl lug. There is also a strong possibility this peening stretched the upper surface of the bar resulting in it bowing up. This would account for the gap at top of breech.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra