My 550 is a straight 550 with a 6 way blade and a winch. My guess would be that it looks like it hooks over the cross bar with the way that 90 degree bracket is. My lift cylinders mount to a bracket with a 90* pin in the cross bar. With that in mind, it doesn't appear to be long enough to reach the rear.
I just did some looking on JD parts at the 450 models. Under the 450c (450 models were basically the same frames up through the G I think). And it appears like it would be a weldable frame mounting for something. The description says they are "side mounting brackets for rear mounted equipment"