Ok. This is what I've found.
The non ridged backing plate has been welded and fitted and does not foul on the frame. The correct alignment is guided by two things. One - the two sprockets lining up and, two - the backing plate not being to far away from the brake arm. If the distance is to far, the lever pulls up on the sliding brass bush when it is tightened and binds. I settled for a .150" spacer between frame and backing plate to achieve the correct distance. I also added a thick o-ring between brass bush and the hole for the brake arm purely as a dirt and water excluder.
I still don't know what model the ridged backing plate is from.