This brings up a very pertinent question. Section 152 has been repealed. Also, later editions of US notes bear the condition: This note is legal tender, good for all debts, public and private.
When demand for lawful money is made, what is the thing (of the resulting trust) received?
Clearly not US notes, since there are none available, and they are limited by statute to $300 million, and they are legal tender.
Clearly not the lawful money of account of the United States, since gold or silver coin is not available.
Clearly not legal tender, since legal tender is being redeemed for lawful money.
PS - Whatever the thing is, it must be as good as gold.