3Position the slots on the TPU’s mounting bracket over the heads of the two mounting screws located at the back of the scanner. Slide the TPU forward slightly until the two holes in front of these slots are aligned with the screw holes at the back of the scanner.
align these holes with the screw holes on the scanner
screw holes on scanner 
position the slots on the TPU’s mounting bracket over the two large screws at the back of the scanner