Canon Color ImageRunner C3380 Reference Manual page 374

10
Load 10 envelopes at a time with the side you want to print on
face up. While loading the envelopes, press down on the right
side of the receptacle to lock them into place.
Claw
Load the envelopes, as shown below.
7
Front
Align the leading edges of the envelopes, and make sure that they are held in
place under the claw.
Claw
7-24
Envelope Feeder Attachment-C2 (Optional)