Mule Note Question

From the PMG website.

Just what is a mule? For small-size notes, a mule is where the plate size of the face and back do not match. This does not mean that the plate numbers are mismatched, such as 4 or 663, but that their physical size of the numbers is mismatched. In the late 1930s plate number size changed from 0.5 mm to 1.0 mm.

Mules are produced in two ways. The first and most common mule is leftover stock of backs that were already printed from the previous series. The second occurs when old plates are reused. The 629 and 637 backs were produced later when the Bureau of Engraving and Printing discovered these two plates were left over and produced notes from them.