Canon Color ImageRunner C3380 Reference Manual page 372

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Envelope Feeder Attachment-C2 (Optional)
Remove all of the envelopes that are loaded in the paper
drawer (several envelopes at a time).
Remove the left guide of the envelope feeder attachment, and
insert the left guide into the appropriate slot for the desired
envelope size.
To remove the left guide, tilt it to the right, and then pull it up.
Loosen the two screws fixing the front width guide of the
envelope feeder attachment.
Left Guide