11x2.5” drum/backing plate ID?

It's a little hard to tell, it looks like the backing plate may be sitting up on the "bump" for the e-brake cable mount. When a BBP, 2.5" wide shoe backing plate is laying flat (ie, the cable mount bump is not holding it up), the measurement you show should be about 1.75". As close as yours is, that's probably what you have. I hang the bottom part of the backing plate off the bench, so the upper 2/3 of the plate has the outer lip flat on the bench to get the 1.75" measurement.

The other thing is, it could still be a 10x2.5" backing plate. The 11x2.5" backing plates have a diameter of about 12", while the 10x2.5" backing plates have a diameter of about 11". So, sometimes people measure the diameter at 11" and think they have an 11" backing plate.

See this post for more measurements Converting rear drum brakes from SBP to BBP