Like mule coins, a mule note is a currency error with a mismatched joining of elements, but in this case, the mismatch is not the full obverse or reverse.
In the case of a mule note, the mismatched parts involve the printing plates with signatures or serial numbers instead of coin dies.
These notes represent a time of change in United States currency. Large size currency notes were phased out in 1928 and 1934, ushering in the era of small sized currency notes. When the notes were changed, many elements of the previous printing system were kept in place. The Bureau of Engraving and Printing wanted to utilize all available resources before making further updates, so many older printing plates were in use until they could not be used any longer.
How are Mule Notes Produced?
There are two variations in mule notes: small-sized currency notes and large-size currency notes.
Mules are produced when there is an overlap in the printing plates used to produce currency notes. In this part of the printing process, one of the two printing plates comes from the wrong group of signers or the wrong series of note issues. This occurs because only one of two plates is changed, leaving the errant printing plate in place.
The plates in question differ by a small margin of size and are distinguished as micro number plates and macro number plates. Micro numbers are 0.6 millimeters in height, and macro numbers are one millimeter in height. One millimeter is small. It is about the size of a pencil’s point or the thickness of 10 sheets of paper stacked on top of each other. The change in size happened for easier legibility.
Identifying a Mule Note
Mule notes were created when micro plates were paired with macro backs and when macro plates were paired with micro backs. If there are two different sized serial numbers on your note, it may be a mule note. Identifying mule notes requires assessing the plate number font size.
Mule notes are more commonly found on small sized currency than on large sized currency, but mules of both sizes have been found. If the plate numbers on the obverse and reverse do not match, it might make the note a mule. This is most common on notes issued between 1920 and 1950 but mule notes from before and after this period have been identified. If you are unsure if your note is a mule note, consult a trustworthy currency book for specific details about mule notes from the series of which your note was issued.
If your currency note has been graded, there will often be a lowercase m at the end of the Friedberg number on the grading slab, but this is not always the case.
Distinguishing Mule Notes from Other Errors
Mule notes differ from double denomination errors in that double denomination errors pair plates from different denominations, while mule notes should have plates from the same denomination.
Another kind of mule note is a note with mismatched serial numbers, which may be referred to as a serial number mule.
