NOT a double jaw at all.

That black "lower jaw" is simply the fish's tongue... prolapsed thru the floor of the mouth thru an earlier injury. Most likely an unceremonious but violent de-hooking at the hands of an irritated troller having to get rid of another small non-retainable king.

You can see where the fleshy membrane between the bony mandible and the tongue was avulsed. Without a way to surgically anchor the apex back to its submental attachment, the tissue simply contracted to form a permanent defect in the floor of the oral cavity.

That's my story, and I'm stickin' to it.