US 100 - Error Or Just Anomaly?

I cropped out the seller's ID

To me, calling this a mismatched prefix error is peculiar. The two prefix letters are J and L, and those (presumably) match in both serials of the note, so there isn't a prefix mismatch. The stray mark in the left-hand serial isn't aligned vertically like a prefix letter would be if there was yet another letter in front of the J; no horizontal spacing at all would exist between whatever the mark is and the J. The stray mark also doesn't look as wide as any of the letters or numbers in the serial, so I'm loath to believe its source is another numbering wheel.

Here is an image from the BEP's website showing an employee manually setting the serialing wheels on a COPE press.


The resolution isn't good enough to see the wheels in detail, but . . .

Here is a still from a YouTube video that shows a closeup of a single set of serialing wheels.


In this image, it is apparent that there isn't another serialing wheel in front of the series prefix wheel, so there has to be another explanation for the stray horizontal mark that is sometimes printed along with the serial number. I feel the grading companies have done a disservice by using the mismatched prefix descriptor for notes with that mark.

As an aside, I didn't know that the characters on the letter wheels are closer together than the characters on the number wheels! The number wheels have 10 characters, 0 through 9. The letter wheels must have more characters -- maybe A through L plus a star or a Z? Since the diameter of all of the wheels is the same, it makes sense that the letters would have to be closer together in order to have room for more of them.