1966 Dart Rear Brake Wheel Cylinders???

Well, I installed a new wheel cylinder from the local parts store, that was for 66 Dart with 10" drums, and front disc brakes. It fit fine, still had a 1/4" bleeder. Have a question, the bolts to the back plate in the wheel cylinder hardly touch the back plate, and the old one was the same. The other wheel has what I call a cap bolt, with a built in washer, that does a lot better job holding the wheel cylinder to the backing plate. Anyone know what is going on here??? Thanks