Lexmark C920 User Manual page 41

If you previously lowered or raised the guide rail plate, the thumbscrews will already be loose.
2
Rotate the rear adjustment knobs to equally align the top and bottom of the finisher with the printer.
To move the top of the finisher closer to the printer, rotate the adjustment knobs clockwise.
To move the top of the finisher away from the printer, rotate the adjustment knobs counterclockwise.
3
Roll the finisher toward the printer until the finisher locks to the docking plate pins.
You hear a click when the finisher locks into position.
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Installing and removing options
Aligning the optional finisher
41