A little more searching on Brake parts reveal that even the 1/2 (W100 series) has standard Drums where as the dart has the Finned ones.

It looks like this dart I have has the Rear Drum Brake setup for the '74 Dodge Monaco. Both Shoes and the big Finned Drums on that year Monaco are what I have on the Dart.

Maybe the Production line was low on Stock Backplates and grabbed from another production line???