Think it must have started with the old Germanicon practice of numbering each individual minute part which (in theory) would only fit the individual weapon. Loewe´s DWM initiated the DIN norm that made parts inter-changeable and any good gunsmith can improve on the reliability of a Luger with the help of a large barrel of mixed parts. But `fully numbered´ is still the benchmark for the market value of a `collector´s´ item. So it´s now just a confidence trick.