The case of the missing 13th amendment to the Constitution
The Iowa Republicans didn’t want the current 13th Amendment banned; they just wanted the “original” one reintroduced for approval. That "missing" proposal was called the “Titles of Nobility Amendment” (or TONA). It sought to ban any American citizen from receiving any foreign title of nobility or receiving foreign favors, such as a pension, without congressional approval. The penalty was loss of citizenship.
It was an extension of Article I, Section 9, of the Constitution, which doesn’t allow a public office holder to receive a foreign title or similar honors without the consent of Congress.
https://constitutioncenter.org/blog/the-case-of-the-missing-13th-amendment-to-the-constitution/