I'm guessing that the missing parts were closest to the surface and exposed to the elements. These rusted away, while the balance of the pistol was protected. By perusing these European and Russian "dig" videos on You Tube, you realize how important the medium in which artifacts were buried affects the condition in which they are found. I've seen MP40s with supple slings and superb condition come out of mud bogs. It is clear that a simple hosing off and lubrication would but them back into operation, while others are nothing but piles of rust.
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