IMHO
Where the rear toggle link receiver ear intersects with the horizontal top of the receiver fork is a very sharp corner. Sharp corners can be points of high stress concentration.
The centerline of the receiver ear rear toggle pivot axle is slightly above the breech block centerline. This slight centerline difference introduce a bending moment into the receiver ear during recoiling of the breech block toggle assembly. The bending moment would be like tearing a sheet of paper as opposed to pulling it in tension.
As a police weapon and over almost a hundred years of use the receiver ear has been subjected to impact forces each time the Luger was fired. During this period apparently a crack was started at receiver ear-fork intersection.
Once the crack was started additional shooting would propagate the crack to ultimate failure.
That's my two cents.
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