US and UK divided by a common language again!
A mending plate is what we sometimes call a mild steel strip with some holes drilled in it for various DIY purposes.Often fixing two pieces of wood together. Usually about 3 inches long.They can be cut down to size.Just cost a few pence at your local hardware store. The mending plate/bracket screws in the same mounting as before on the dropouts.It just acts as a short extension to clear the toggle chain on the 3X7, I have not fitted one on the 3X9 black bike. Its a 5 minute job if you opt not to use the sram click box.