Mule notes are created when there are changes to the note's plate number, and occur on large and small notes. Mules are produced in several 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.
Some mule notes are scarce, while some are not. Sometimes the non-mule note is scarcer than the mule note for a given series. Mules are an intriguing part of collecting small size United States paper money.
Large Note Mules
Beginning in 1921 when Frank White took office, new back plates were created for notes being printed and the location of the plate number was changed — Detailed information can be found in The Comprehensive Catalog of United States Large Size Star Notes by Doug Murray.
Small Note Mules
Beginning in 1938 the plate number size changed from 0.5mm (termed "micro") to 1.0mm (termed "macro"). A small note mule is a note that has a micro plate number on one side and a macro plate number on the other. For $1 Silver Certificates this transition for the obverse side was between Series 1935 and 1935A. For all other Federal Reserve Notes and higher demonination Siver Certificates, the transition was between Series 1934 and 1934A. For $2 United States Notes, it was for Series 1928C and 1928D. For $5 United States Notes, it was for Series 1928B and 1928C.
Most small size mule notes have different size front and back plate numbers, but that doesn't necessarily make them mule notes — For example, the Series 1981 $1 Federal Reserve Note Back Plate Number Engraving Error with a 0.8mm back plate number is not a mule note.
A mule note is simply a Series of notes that uses back plates from a differnet Series. Mule notes can be produced by using back plates from a prior Series — The Series 1963 $1 Federal Reserve Note, for example, has new face plates, but uses back plates from Series 1957 B $1 Silver Certificates. Mule notes can also be produced by using back plates from a newer Series — Series 1934 $5 Federal Reserve Notes with back plate numbers 939 are Series 1934 faces with Series 1934 A backs!
During the transition to all macro plates, both micro and macro plates were in use. Usually these different plates were side-by-side on the same press. This occurred because the BEP had a standard economic policy of using up obsolete plates rather than scrapping them. Whenever micro faces are paired with macro backs, or macro faces are paired with micro backs, a mule is produced.
Some scarce mules to look out for include: