The fact that the FRN omits that it may be exchanged for lawful money is an admission (through omission) that it is knowingly unlawful.

https://web.archive.org/web/20221109030937/https://famguardian.org/Subjects/LawAndGovt/LegalEthics/LegalVLawful.htm

Lawful. In accordance with the law of the land; according to the law; permitted, sanctioned, or justified by law. “Lawful” properly implies a thing conformable to or enjoined by law; “Legal”, a thing in the form or after the manner of law or binding by law. A writ or warrant issuing from any court, under color of law, is a “legal” process however defective. See legal. [Bold emphasis added]

Legal. Latin legalis. Pertaining to the understanding, the exposition, the administration, the science and the practice of law: as, the legal profession, legal advice; legal blanks, newspaper. Implied or imputed in law. Opposed to actual

“Legal” looks more to the letter [form/appearance], and “Lawful” to the spirit [substance/content], of the law. “Legal” is more appropriate for conformity to positive rules of law; “Lawful” for accord with ethical principle. “Legal” imports rather that the forms [appearances] of law are observed, that the proceeding is correct in method, that rules prescribed have been obeyed; “Lawful” that the right is actful in substance, that moral quality is secured. “Legal” is the antithesis of equitable, and the equivalent of constructive. 2 Abbott’s Law Dic. 24. [Bold emphasis added]

Index of Topics from Abbott’s Law Dictionary (1893)

https://web.archive.org/web/20221231185206/http://ecclesia.org/lawgiver/defs.asp

Lawful in this case is gold or silver.