Brother HL-3450CN - Manual (Page 164)

CHAPTER 6 OPTIONS
6-8
4. Remove the Back caps (2 pieces)
of the Lower Tray Unit. Push
down lightly on the caps and slide
them to the rear of the printer to
remove them.
Fig. 6-12
5. Use the handholds of the Duplex
Unit to lift the Duplex Unit up.
Fig. 6-13
Note
Ensure that the Duplex Cover Top Assembly is aligned and latched
straight with the lower part of the duplex unit.
6. Insert the fixing pins at the sides
of the Duplex Unit into the
installation grooves in the Lower
Tray Unit.
Fig. 6-14
Note
Make sure that the Rear Cover of the Lower Tray Unit is closed.