[Smokin' Joe]

Yes United States Notes had been issued in the past; a direct monetization of debt. Silver certificates could still be exchanged for silver coin (.77 oz/dollar at 90% Ag content), which in and of itself wasn't so unusual while JFK was president (it only stopped being produced at the mint after 1964).

The Federal reserve notes initially said "This note is legal tender for all debts, public and private, and may be exchanged for lawful money at the United States Treasury or any Federal Reserve Bank."

After 1965, Silver Certificates and United States Notes disappeared from circulation, along with the vast majority of 90% silver coins and 40% silver halves (from 1965-1970), leaving FRNs and copper clad coins.

[Cboldt]

-- The Federal reserve notes initially said "This note is legal tender for all debts, public and private, and may be exchanged for lawful money at the United States Treasury or any Federal Reserve Bank." --

There were a few iterations:

1914

This note is receivable by all national and member banks and Federal Reserve Banks and for all taxes, customs and other public dues. It is redeemable in gold on demand at the Treasury Department of the United States in the city of Washington, District of Columbia or in gold or lawful money at any Federal Reserve Bank.

1928A

Redeemable in gold on demand at the United States Treasury, or in gold or lawful money at any Federal Reserve Bank.

The version you noted, which referred to "lawful money" (whatever that is) but not to gold, appeared in 1934. That phrase was replaced in 1963 with the words we see today, "This note is legal tender for all debts, public and private."