If you stick the tape measure through the hole it should hit the axle retainer plate. Record that number, then measure the backing plate thickness and axle plate retainer. Ideally you measure from the outside face of the axle hub to the axle tube flange to get the brake offset. When you're talking to Currie tell them your first measurement (from axle flange face to retainer plate) and they should know the rest.

I pulled that picture off the internet, I couldn't find my 9" axles quickly.

 

Okay. But, in the pic you posted, if you were speaking to currie, what off-set does it show?

 

From the picture I would tell them it's 2 1/4" to the retainer plate, then ask what size will work with that.