Does this look right?

There are different width backing plates for different width drums. I believe there was 1 3/4 " for the 10" drum and 2 1/2" backing plate for the 10 1/2" drum. This I am not sure of, but you get the idea. maybe someone else will chime in to confirm. I believe 10" is standard for A-body and the E and B-Bodies had the 10 1/2" drum availible on most.