If one wants to be pedantic, hallmarks are simply marks of purity, and consist of fineness marks, assay marks, sponsor marks, responsibility marks and control marks (and probably others depending on region). The U.S. uses kt and k, depending on space available.Most nibs are not hallmarked at all - simply stamped with a fineness mark (usually 14k/585, 18k/750 or 21k/875). Whether they show k or ct depends on where you are, with ct usually being used in the UK and Australia and k in the US, Canada and much of Europe.
So gold nibs are hallmarked with purity (fineness) at a minimum as international convention. The "extra" marks that provide further assurance (depending on country) are also hallmarks.
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