> what identifying characteristics did the money that ended up in Iran have?
The money was flown into Iran on wooden pallets stacked with Swiss francs, euros and other currencies as the first installment of a $1.7 billion settlement resolving claims at an international tribunal at The Hague over a failed arms deal under the time of the Shah.
https://www.cnn.com/2016/08/03/politics/us-sends-plane-iran-400-million-cash
To return the principal, the Treasury, working with the Defense Finance and Accounting Service (DFAS) and the Federal Reserve Bank of New York, made a $400 million wire transfer from DFAS to the Swiss National Bank. The $400 million was then converted into Swiss francs and withdrawn in franc banknotes, which were transferred to Geneva. On January 17, the banknotes were disbursed to an official from the Central Bank of Iran.
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Since the Judgment Fund does not allow the processing of individual claims of amounts over ten digits, the agreed upon interest - $1.3 billion - was split into 13 claims of $99,999,999.99 and one claim of the remaining $10,390,236.28. These amounts were transferred from the Judgment Fund to the Dutch National Bank, where they were converted into euros and withdrawn in euro banknotes. The Dutch bank then disbursed the notes to a representative from the Central Bank of Iran.
https://www.brookings.edu/articles/the-united-states-iran-and-1-7-billion-sorting-out-the-details/