Glock was a new guy, and certainly for liability protection if a problem arose from swapping in early years, and perhaps also original Austrian contract specified, needed or not. I am not enough a Glock historian/fan to know what earliest marks comprised, have seen, but do not recall, think last several numbers initially? Proof mark has changed/relocated, and numbering may even be part of proof law there, which we lack in the USA...my Pedersoli muzzle loader has a proof on each barrel AND breach plug.
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Cadent a latere tuo mille, et decem millia a dextris tuis;
ad te autem non appropinquabit.