Look at the gun from the side, noting that right angle of the frame on the rear. Notice that the rear isn't a perfectly straight edge from bottom to top, but slightly bulges back toward the top edge. That "hump" is designed so that the main rear toggle pin can not work its way out during operation. And it was a feature distinctive to Mauser mfg. Lugers in the very late 30's. And it helps us ID a late 30's Mauser frame.
Hope this helps.
dju
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