I posted this note with “redeemable in gold” in the title and explained that, even without a gold clause like the ones on GCs or FRNs, these notes could be exchanged for gold between 1879 and 1933. So 1928-plain $2s and $5s were redeemable in gold when they were initially issued.
However, one of the moderators deleted the post. So I’m reposting it with a clear explanation of how these notes became convertible. It’s actually partially mentioned on Wikipedia.
Legal Tender/United States Notes were first issued as fiat money in 1862 to fund the Civil War. Unlike their predecessor, Demand Notes, they couldn’t be used to pay customs duties, which were the main source of tax revenue at the time. Due to these limits, they immediately traded at a discount, and in July 1864, $285 of greenbacks was needed to buy $100 in gold coin. After the end of the Civil War, the rate stabilized at around $140 to $100, meaning they traded at roughly a 30% discount.
When the US purchased Alaska for $7.2 million in 1867, the amount was expressed in gold , so it was roughly $10 million in greenbacks. The greenbacks appreciated further after late 1869, and in 1875, Congress passed the Specie Payment Resumption Act, which stated that they would become redeemable in gold after 1879. https://www.gold.org/sites/default/files/documents/1875jan14.pdf
They actually became redeemable on that year and started to be accepted for customs duties. However, the official clause remained unchanged, so no gold clause was added, and it continued to state that they weren’t legal tender for tariffs.
After May 1878, the amount of these notes was fixed at $346 million, so all newly issued notes after that were simply replacements for retired notes. Their gold backing initially fluctuated, but since 1900, it remained fixed at $150 million+ worth of gold valued at $20.67/oz. In conclusion, United States Notes were redeemable in gold coin and remained at par with gold until March 1933.