Apple LaserWriter 12/640 PS - Manual (Page 114)

5 Extend the tray.
Installing the face-up output tray on the optional duplex printing unit
1 Line up the tabs on the bottom edge of the face-up output tray with the holes in the back
of the duplex printing unit.
2 Gently push down on the face-up output tray to insert the tabs into their receiving slots.
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