These are the areas that receive the stress of the pressure from firing. The sharp corners are susceptible to cracking from excess stress or fatigue. The inclined area is the actual locking surface that takes the pressure of firing. The Toggle knobs don't really do anything until the pressures have dropped and the upper receiver has travelled to the rear to start the unlocking sequence by the knobs striking the frame ears and forcing them upward to "break" the toggle joint.
https://forum.lugerforum.com/attachm...1&d=1687820755
https://forum.lugerforum.com/attachm...1&d=1687820755
https://forum.lugerforum.com/attachm...1&d=1687820755