10 inch drum backing plates

Measure the backing plate diameter. 10 7/8 is 10 inch backing plate and a little over 12 inches is 11 inch backing plate. My guess is those are 11 x 2 backing plates. It has been posted on here several places/times as follows. Dana is correct in the way it is measured but the way you measured it ,is also correct.
1 1/2"=11 x 2
1 3/4"= 10 or 11 x 2 1/2
2" = 11 x 3