Lexmark C510 Guide D'installation page 24

Redrive
assembly
Set up the optional duplex unit
18
3
Insert the bottom of the duplex unit
into the lower opening as shown.
4
Pull up on the top of the duplex unit
as you swing it up over the top of
the printer, and then press it down
so that it locks fully into the upper
opening.
When the duplex unit is mounted
correctly, there is a small and even
gap between the duplex unit and
the top of the printer, visible from
the front of the printer.
5
Open the duplex rear door and pull
down the redrive assembly.