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[dready]

Originally Posted by CamaroDMD

Mules are much more common with paper currency. On coins, there are unbelievably rare and in many cases are fakes.

This is courtesy of The Heritage Auction site:
Mule Notes
Mule notes occur on both Large- and small-size notes. On Large-size notes beginning in 1921 when Frank White took office, new back plates were also created for notes being printed, however, 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.
A small-size mule is a note that has a micro plate number on one side and a macro plate number on the other. Micro numbers measure 0.6mm and macros measure 1mm high.
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 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. Here is an example of a mule note that I received as a prize for winning a contest held by RickieB. Thanks again Rickie..................................John