First $1 FRNs Green Seals had Blue Seal Silver Certificate backs - post your 1963 Mules !
The small size $1 Silver Certificate Blue Seal printing concluded with the 1957-B series back plate 447 on the Y-B block as the transition to 1963 $1 FRN Green Seals began. To save expense printing the new Green Seals (and because they figured nobody would notice) the BEP continued using the old back plates from the 1957-B $1 Silver Certificates until they wore and out and were replaced. During this time there is a brief rarity possible to find.
These Green Seal 1963 $1 FRNs with back plates that number 447 and lower are actually $1 Silver Certificate Backs.
We call the mismatched front from one series and the back from another series "a Mule" just as in the animal kingdom where it is derived from two different parent breeds.
Here are a few of my Mules, bp 435, 436, 446 and 447 (I vaguely recall somebody looking for these specific bp s?)