What Caused This Printing Artifact?

The following image is from an early 20th century U.S. wrapper. The embossed stamp was produced on a letterpress (Huckins rotary) press.

I am interested in how the heavily inked line across the middle of the stamp was created. Other examples are known with the horizontal line at another position (i.e., not directly across the middle), but, in general this is a rare artifact. Any help will be appreciated.

Hello,

Letterpress was my first experiment.

I don't know the kind of printing press was used, but I think that it may be a stop of the printing press which makes a heavy line of ink on the inking roll, and after on the printing die. The stops are not always at the same place.

I may be wrong, but I think so.

Papy24

Thanks Papy24, sound like a reasonable scenario.