Oki OKIPAGE 24DX Service Manual page 291

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.
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.