This is a highly technical misprint that is usually only spotted by collectors with a trained eye. The misprint on these bills occurs on the serial number on the upper right hand side of the bill. The M that begins the serial number is actually an upside down W. The M on the bottom left serial number is correct.
M - has completely vertically lines.
W - has slightly angled lines.
These are not especially valuable. However, most people who collect silver certificates like an example of the misprint. Send us a picture of what you have and we can make an offer. firstname.lastname@example.org