Oki OKIPAGE 24DX Service Manual page 141

Installing Duplex Unit
(1) Hold the LEVER-Lock (Dup) in horizontal lock position.
Then insert the Duplex Unit along the groove, up to accord both rear faces between printer and Duplex Unit.
Note:
Printer and Duplex Unit will connect automatically with built-in connector.
(2) Lock the Duplex Unit to the printer by turning the LEVER-Lock (Dup) to the next locking position as below.