"The Federal Reserve is aware of several scams involving high denomination Federal Reserve notes and bonds, often in denominations of 100 million or 500 million dollars, dating back to the 1930s, usually 1934. In each of these schemes, fraudulent instruments are claimed to be part of a long-lost supply of recently discovered Federal Reserve notes or bonds.
Fraudsters often falsely claim that the purported Federal Reserve notes or bonds that they hold are somehow very special and are not known to the public because they are so secret. Fraudsters have attempted to sell these worthless instruments, or to redeem or exchange them at banks and other financial institutions, or to secure loans or obtain lines of credit using the fictitious instruments as collateral.
The Federal Reserve has never issued any bonds or notes with coupons attached"
https://www.newyorkfed.org/banking/FRBNY_archived_fraud_alerts.html