I mounted a 3 point Truck (fender mount) mirror over the plunger case on my 575 NH just for that reason. My power units have mirors but I can't see a mistie (don't happen often) until it's 3 cycles past the quarter turn and then I have to stop, move bales, turn the machine around and rebale the miss. PITA. With the 575, there is a whole lot of machine back there and there is no way you can see what's happening.
I angled the mirror so I can see the knotter and the bale as it's packed and twined. I just watch in the tractor mirror (left hand) and look at the bailer mounted mirror ocassionally or between sips of a cool beverage......:licking:
The spot mirror allows me to watch the bales as they get tied and stop if I have a problem, right away.
Saves time, especially in wheat straw in long windrows. I bale contract so time costs me money as well as mistie'd twine.