If you compare a WW1 Luger with a later Mauser, you will note that a small protrusion was added at the very back, top of each lower receiver ear, (above and behind the safety on the left receiver ear). This was added because it was apparently found that on the older guns, the upper receiver and toggle train could come back far enough during recoil or brisk charging of the pistol to expose the entire diameter of the rear toggle axle, allowing it to come out far enough to bind the whole system. The added "hump" to each receiver ear prevents this from ever happening.
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