Although mostly hand fitted, Lugers are surprisingly resilient when it comes to mating MAJOR subassemblies, (i.e. the entire upper from one pistol with the entire lower of another. They usually work. That is perhaps why they survived in this condition. If they had not worked upon assembly, the troops who accidentally swapped them would then try to find their correct missing parts.
Where you run into trouble is usually with errant small parts, principally the sideplate, sear, bolt, etc.
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