Those are ‘special issues’. They are complying with the ‘law’ that was passed during the late 1990s when all the rage was about a Social Security ‘lockbox’. Again this is the con they pull (political members).
The SS Administration has to know they are complying with the law because they are bureaucrats and are not sophisticated enough to think as clever con artists. The law is subverted outside their realm. They are not allowed to buy treasuries on the open market, they receive a ledger entry as part of their compliance.
There is no interest paid on the special issues. It is all compounded into an unfunded liability which is not part of government operations.
The trillions of annual debt that the Obama Administration has embraced with approval from Congress does not include ledger entries of special issues between the Fed and the SS Trust Fund. The trillions are derived from open market purchasing.
The open market purchases are today mostly domestic purchases from Fed policy to member banks (including mutual funds) to bond traders. This is the debt that is in focus.
There are far more trillions that go offshore but which are not on the books that the Fed presents to Congress.