11" Backing Plate Differences

[RogerRamRod]

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.

 

[Fran Blacker]

Measure the width on the drum where shoes contact. Subtract thickness of spacer that should tell what backing plate you have and drum width. That's assuming drums didn't with spacers, ditch those. Put a straight edge across axle face and measure to backing plate lip and add 1/4" to 3/8".