The coolest solution I've read about to note the producer is to emboss a hologram in the metal, though perhaps with enough care this too could be faked or copied:
Edit: And of course put the info on block chain.
http://www.sunshinemint.com/TechnologyEnhancements.aspx
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Geiger Edelmetalle , on their Geiger branded stuff, uses a a UV safety coating but it will rub off with enough handling (which is why many of us do not remove the shrink wrap they put on their bars), they also individually assign serial numbers that can be tracked online for gold and silver bars starting at 1 gram
https://www.geiger-edelmetalle.de/en/about-geiger/news/geige...