11" rear backing plates

[tals]

I have 2 sets of 11" rear backing plates that I need to id. If I put a straight edge across the axle flange and measure to the shoe pad, one set is 3/4" and the other is 1 1/4".

Anybody got info on this? Maybe 2" and 2.5"? Or maybe 2.5" and 3"? Truck?

Thanks to all.