The Deutsche Bundesbank has a long list of every single gold bar in their possession (including those currently stored in GB and USA), including their weight (to 0.1 gram) and purity (at least 995/1000 as far as I can tell).
https://www.bundesbank.de/resource/blob/743058/9869caef634ce... - Federal Reserve Bank of New York starts at page 2016 (PDF:2019)