As I mentioned earlier minor redesigns (generally one of the signatures) resulted in the Series Date remaining the same but a letter was added starting with A. But major redesigns resulted in a change in the Series Date. The Series 1934 was the result of changing the design of the face. The back remained the same as the 1928 Series. The Series 1935 was the result of changing the design of the back. The front remained the same as the 1934 Series. Series 1934 Series 1935 Series 1935 Mule Mules, like with coins, are faces and backs that weren't meant to be paired. But unlike coins, currency mules are not readily obvious. In this case the sizes if the Check Numbers were different. Check Numbers (which I call plate numbers) are in the lower right on the face and back. The size difference is small (1 mm vs .6 mm) but like similar differences on coins, they are significant to specialists. In the case of the 1935 Mules it is 1935 faces matched 1935-A backs.
