1867
Greenbacks below gold
In 1867 United States Notes are legal tender. The Treasury is not redeeming them in gold on demand. The Act that names a resumption date is still 1875.

The Legal Tender Act of February 1862 authorized United States Notes as legal tender. The Treasury would not redeem that paper in gold on demand. In the market the notes traded below gold.
That is still the regime in 1867. The Specie Payment Resumption Act, which names 1 January 1879, has not been passed. It is 1875.