"When an electronic flash is connected to the hot-shoe, current moves through the X contact. So be sure to put the safety cover supplied with the camera on the X contact so that you will not receive an electric shock."
It's kinda hard to get an electric shock with just 6V on the contacts is it? So unless I'm misreading it, they must be assuming a higher voltage there.
Sounds like Mamiya recognizes some flashes have high voltage that can run from hot shoe to x-sync and doesn't give a warning except for human shock