Interest bearing notes were an early type of money originally issued during the Civil War. They were a little bit like a savings bond. You could spend them like cash, but at certain dates they earned interest and became more valuable. Today they are exceptionally rare. At a time when all other $20 bills were worth just $20, interest bearing notes might be worth $22. So there was absolutely no incentive to not cash them in. Abraham Lincoln is on the right hand side of the bill. There are some fakes of this issue floating around today. Be careful when considering a purchase.