All of the above is correct, of course it doesn't explain why You have a set of 11x2" backing plates on an 8.25" rear, which never happened from the factory. Someone
may have swapped up to them because it's what they had laying around, and replaced the OE 10x2.5" units with them..........really the only explanation I can think of.
72bluNblu's sticky in the drivetrain forum has the dimensions, is this an A-body rear? When I swapped an 8.75 from a '69RR into The ex-PoPo Dippy, I pulled the axles
& hung the whole brake assy. on each side from the 8.25, then put the 11x2.5's right on the RR rear. Less likely but possible someone slipped that rear into an older B
and did the same w/their brakes...................but the spring pads would be different unless they moved the leaves in.