Not sure if this is a scratched plate or ink on top of the image. I could be wrong but, perhaps a plate that wasn't wiped clean? The mark starts out narrow, widens, narrow again, then smooths out. 70% of the way across there is a heavy dark mark going up, following a hair line. Just above the left end there is also a heavy dot of ink.
Edit: also narrows again about 45% of the way across.