Those are 6x4.5 frames, so the film was moving top to bottom, not left to right, based on the images, correct? The hard line look reminds me of a leak that is happening outside of the exposure area. As if the film is wound tight to the takeup or feed spool and the leak is happening as the film comes off of the spool; the line is where the film is hitting the backing paper.
Put the negatives back in the camera as shoot and look at the areas where the film goes onto/off of the spools, with the leak happening on the frame before or after actual exposure (I don't know the Mamiya film backs well enough to be able to tell if the leak is on feed or takeup, but you'll see right away when you put the film in an insert which side would have the exposed area actually out in the open).