ISIN
ISINs are twelve-character ISO 6166 keys whose nine-character NSIN is allocated by national numbering agencies.
The International Securities Identification Number is the closest thing finance has to a global primary key for a specific issue. ISO 6166 defines its twelve characters: a two-letter country prefix from ISO 3166, a nine-character National Securities Identifying Number allocated by the relevant national numbering agency, and a final check digit computed with the Luhn algorithm. For US and Canadian securities the NSIN is simply the CUSIP, which is why a US ISIN reads as a familiar identifier wrapped in a country code and a fresh check digit. When two datasets disagree at the join, validating structure and check digits is the fastest first audit you can run.