Backing plate info

Murray said
You may have one front and one rear. One plate has a wheel cylinder piston retainer. I think these were more common on rears.

The attaching bolt pattern is quite different on front plates vs rears. Center height and hole diameter are also substantially different.
Front

Rear

OP, my best guess is early vs late model backing plates- somewhat different but functionally interchangeable. I'm sure someone will correct me if I'm mistaken.