Apple LaserWriter 12/640 PS - Manual (Page 320)

Checking the optional duplex printing unit
1 Remove the the duplex printing unit tray to see whether the jam is in the tray area. If so,
gently pull out the jammed paper.
Hint: It may be difficult to view paper jams in the duplex printing unit tray
area because paper jams may occur toward the back of the tray area. Look for
wrinkled or curled edges indicating a paper jam.
2 Open the duplex printing unit’s door to see whether the jam is around the paper guide
for the duplex printing unit. If so, gently pull out the jammed paper.
295 Fixing Paper and Image Problems