Oce OP33 Reference Manual page 271

5
Pull out the left guide, and remove all envelopes already loaded in
the drawer.
6
Loosen the two screws fixing the front width guide of the cassette.
7
As shown below, push down on the envelope receptacle, and move
the front width guide so that they are aligned with the mark for the
desired type of envelope.
Envelope Feeder Attachment-B1
7
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