11" Backing Plate Differences

I have 11 x 2-1/2 drums. I'm under the impression that there are different Backing Plates for 2" -vs- 2-1/2" shoes/drums. Do you know if that's correct?

Somebody in the past put a spacer under the drum. If I remove the spacer, the drum scrapes against the outer lip of the backing plate.

I'm not really sure what to measure. I currently have 11" backing plates, so I'm assuming there is a different "Depth" to different backing plates.

Does anybody know if there is a part number usually located in a usual spot under all the crud where I should be looking? Or do I need to remove them to lay them on a flat surface to get a measurement?

Thanks in advance for any info you can provide.

I posted this on B-Bodies as well since this is a B-Body rear I'm using.