I don't know the "official" production breaks on the rear brakes, but, while they did use Basically the same set-up, I know for a fact that the brake shoes changed 3 or more times from 71-79. I know that the alignment pins and hardware mounted differently, so i assume that the backing plates were different?

I know this after getting two sets of wrong rear brakes for a '72 we had at the shop a while back before the third try that revealed the correct ones.
The differences are subtle, but they are different. Again, i don't know for sure about the backing plates though.